I've been thinking for a while that having Detect Magic as a spell doesn't really make a lot of sense. In my mind (and in a lot of the fantasy literature), when arcane powers are used, they cause disturbances felt by those with arcane affinity of some sort. That is, when a wizard casts a spell, or someone invokes a powerful ritual, or someone taps into a ley line, then those who know magic may have a sense of that thing happening. Because they traffic in arcane power, they somehow can feel when someone uses such power, especially if it's a powerful effect. The distance at which one can feel this effect could be debated, but I'd argue that more powerful effects can be detected a greater distances. So, a little cantrip might go virtually unnoticed, but the spell, Lightning Bolt, would be much easier to detect. A complex and powerful ritual requiring a bunch of wizards would be like a beacon (and in fact, when we think about what might be summoned through such a ritual, it probably very much is a beacon of sorts). A spell that has been cast with more power might be more detectable than the same spell cast with less power.
Fantasy RPGs, in my experience, do a poor job of modeling that element of the fantasy literature. Depending on the system we're paying, if we want a wizard to know if, for example, an object is magical, we cast Detect Magic. If we want to know what kind of magic it is, then we cast Identify. Surely, there are different spells in various systems that are used to achieve these sorts of effects. My point, though, is that wizards actually have to cast a spell to detect this stuff, let alone to identify it.
I'll use DCC RPG ruleset as the basis for what follows. Let's begin with a problem.
What if you're a wizard standing in front of a closed door. You want to know if there's any kind of magical trap. With the rule-as-written, you might have to cast Detect Magic. Depending on the roll for spellcasting, assuming it's successful, you could learn that, yeah, it's magical. Or you might learn how magical it is. Or you might even get some (or a lot) of information about the nature of the magic. However, you can't just be in the presence of the magical trap and say, "Hmmm... this shit's magical, and I can tell that simply because I'm a wizard, and we know that kind of stuff. Maybe I don't want to fuck with it until I figure out what it is and how it works." Why can't wizards just do that?
Option 1: The magic has been hidden in some way (e.g., weaves were inverted to hide them).
Option 2: The magic is of a type not familiar to the wizard, as magic (e.g., divine or demonic power of some kind).
Option 3: The magic has not yet been activated (i.e., some element of the spell has not been completed).
I think all of those options are reasonable explanations.
With Option 1, then, perhaps instead of (or more probably, in addition to) Detect Magic we should have a spell called Hide Magic. Also in that line of thinking, an Identify Magic spell might be complemented by a spell designed to promote misidentification of magical effects. It looks like a simple Hold Portal, but actually is a Fireball spell triggered by opening the door.
With Option 2, there would have to be various flavors of magic in the world, some of which you'd know and some not, and alignment might be a part of that. Your cleric could, for example, tell you if something seems holy or unholy, because she's a freakin' cleric. You need that kind of caster to detect that kind of magic. That's just one example of things in this genre. Another: Maybe a fire wizard would be better able to detect elemental magics, but other types are harder to detect and/or identify.
With Option 3, spells are done in stages. Maybe someone has rigged a door with a rune of some sort. Maybe it's visible or maybe it's not. More importantly, the rune must be activated in some way. Maybe there's a needle trap with a silver needle which must shed some blood. Maybe there's a piece of chalk fixed to the bottom edge of the door. When the door is opened, it will complete the rune and trigger the spell effect. In either case, the spell is not detectable, because it's not yet triggered. There is no arcane power to be felt, at least not yet. There are, however, outward signs of the making of such powers: chalk marks around the doorway or on the floor; a trap with an unusual component (a silver needle engraved with weird symbols, which could be found by a thief, perhaps).
My point is this: I think Detect Magic needs to be expanded a bit. Wizards are magical beings because they are able to cast spells, and/or as a result of such casting. They traffic in the arcane. So, when a strong spell effect is generated, they probably can detect it, unless something is done to hide the effect. Here's a possible mechanic to model this, using the DCC RPG spellcasting roll as its basis.
When a spell is cast successfully, the number generated for the casting roll is proportionate to its power. Thus, the higher the roll, the more likely it is to be detected. My guess is that the person doing the detecting must be a wizard or elf, someone who uses such magic. The judge could look at the base success for that spell, subtract that number from the casting roll, and then roll under that number, with modification by Luck or Intelligence.
Example: The party of adventurers is attempting to enter a wizard's tower via a locked entrance. The Wizard (let's say she's 3rd level and has an Intelligence attribute of 15) casts Knock, getting a roll of 13 on her d20, plus 3 for caster level, plus 1 for the Intelligence mod, for a total of 17. The base success for that spell is 12. 17 minus 12 equals 5. The wizard who owns the tower would have a chance to detect such magic, automatically. That base chance would require her to roll under 5. The roll would be modified, as was the casting roll, by caster level and Intelligence modifier. So, a 5th level wizard with an Intelligence of, oh, 16 would have modifiers of 5 for caster level and 2 for Intelligence. Those would be added to the "5" result for a total of... 5+5+2=12. That would be the "roll under" number to detect the effect automatically.
Now, certainly, range might be a factor. If one is just too far away, it would be difficult to detect magic. That sort of thing still needs to be worked out, of course. Similarly, one could take steps to hide magic use. Maybe you have an artifact that does so. Maybe you cast some other spell that obscures your magic use from prying minds. Those things will be put aside for discussion at some later date. Now, let's look at the spell itself: Detect Magic.
In DCC RPG, the Detect Magic spell is a cleric spell, and is a combination of detection and identification. It allows both detection of magic and, depending on the results of the spell casting, a determination of its potency, its effects, and so forth. While I strongly disagree that, as a clerical spell, the wizard should cast it at a penalty, I'll leave that aside for now. The spell itself, even with the addition of the mechanic I've laid out, above, would still be useful. Sure, the wizard in the tower might have "felt" somebody using magic nearby, but she would still need to figure out some things about it. Maybe Detect Magic would be useful in that regard, so she could cast it. However, she would not just be blissfully unaware that powerful magic had just been used nearby, and that she might actually need to summon the flying monkeys to deal with whatever ruffians might be lurking nearby, or already on the premises trying to kill her and take her stuff.
Further, Detect Magic could be reconfigured as an inversion of the spell-as-written, or it could be used as a kind of saving throw, where the DC is the casting number of the spell she is trying to detect (as is the case with Sleep and other wizard spells). So, let's say the wizard casts Hide Magic before casting whatever other spell she wishes to cast. She rolls a 20 result, say. Perhaps another wizard nearby would have to make a save against a DC of 20 (modified by caster level and Intelligence mod) in order to "feel" whatever that spell was meant to hide.
Example: Chlorimina the 3rd level wizard is attempting to enter a rival wizard's tower, and is accompanied by a thief and a couple of warriors. She finds that the entrance is locked, and the thief can't seem to get it open. Maybe it's locked with a Hold Portal spell. She wants to cast Knock, but knows that her rival, Dementia (a 5th level wizard) would detect that. So, she casts Hide Magic first (which, by its nature, is not detectable by normal means). Her casting roll is 13, and is modified by +3 for caster level and +1 for Intelligence mod, for a total of 17. Dementia, the rival wizard, must save against a target number of 17. So, when Chlorimina casts Knock to open the door she wishes to open, the Judge rolls a d20 for a 9, modified by 5 for caster level and 2 for Intelligence for a total of 16. This doesn't beat the 17 on Chlorimina's Hide Magic casting, so Dementia fails to detect that her rival has used a spell nearby. Her enemies have penetrated her sanctum, but she has no idea that they have. However, should Dementia cast a Detect Magic spell, it may be that she would be able to determine that something has happened, and, depending on the results of the roll, its direction, nature, power level, etc.
Anyhow, these are just a few thoughts on the matter. I'm not yet satisfied that this mechanic is usable in play. I'm sure I'm probably missing something important. However, it does capture to some extent both what I see as problems with Detect Magic as-written, and some ways in which those problems might be addressed. What do you think?