One of my players has a constant detect magic and is claiming that being able to literally see magic should give a bonus to Spellcraft to determine what spells are being cast. Is this reasonable and if so how much of a bonus should I give?
Detect Magic does not provide bonuses to Spellcraft checks, but it does allow you to use Spellcraft in additional ways.
First off, Detect Magic does not allow you to "literally see magic." It allows you to "detect magical auras," which is not the same thing. Your player may be conflating it with the related spell Arcane Sight, which does allow you to see magic, but for the purposes of this question this is irrelevant - neither spell provides bonuses to Spellcraft checks.
The difficulty of identifying a spell as it's being cast is given in the description of Spellcraft skill. It says:
Spellcraft DC 15 + spell level - Identify a spell being cast. (You must see or hear the spell’s verbal or somatic components.) No action required. No retry.
That's all the rules have to say on the matter.
The Detect Magic spell, meanwhile, doesn't interact with spells as they're being cast at all. It does what is says it does, allows you to get information about magical auras. To do so, you must spend at least one round studying an area or an object, which precludes getting any information about something that happens outside of your turn, like another creature casting a spell. Furthermore, it's unclear whether "casting a spell" is even something that would give off a magic aura; the spell says you can detect a "Functioning spell," but this is generally understood to refer to ongoing spell effects.
There are two ways that Detect Magic does interact with Spellcraft:
- After studying a magical item or area for 3 rounds, you can make a Spellcraft check (DC 15 + spell level, or 15 + half caster level for a nonspell effect) to determine the school of magic associated with any magical auras you've detected. This might allow you, for instance, to recognize that the rock wall blocking your path is actually an illusion.
- One of the use cases given in Spellcraft's description is to "Identify a spell that’s already in place and in effect." In order to do so, you "must be able to see or detect the effects of the spell." Some DMs might rule that detecting a spell with Detect Magic counts as being "able to...detect the effects," and therefore allow you to roll Spellcraft to identify an otherwise imperceptible ongoing spell effect once you notice its aura using Detect Magic.