What is the mechanic of the check when using detect magic, Greater to identify magical signature?

Question

Detect magic, greater says:

You can recognize this signature if you succeed at a Spellcraft check when later identifying a spell to determine whether or not that spell was cast by the same individual.

I am confused what is the mechanic for the DC and other functions for identifying spells signature features:

• isn't it supposed to be a knowledge (arcana) to recognize spells currently in effect?
• is the DC calculated using spellcraft skill's "identify spell as it being cast" math or the one presented previously on the spell regarding "identify last spell cast by creature" effect of the spell?

Answer 1

You need both skills.

This somewhat confusingly-worded section pertains to your ability to recognize the "signature" of a spellcaster the next time you see it. Here's the full text of that section (emphasis mine):

Finally, you are able to locate and analyze the signature flourishes in a magical aura that allow you to match a spell to the person who cast it. In order to find these identifiers in a spell's aura, you must spend 1 round focusing on that spell in particular, and succeed at an opposed Knowledge (arcana) check against the caster (or a Knowledge [arcana] check with a DC equal to 15 + the spell level if the caster wants her work to be identified and emphasizes these unique elements rather than obscuring them). Once you learn a caster's set of identifiers, you can remember them as easily as a face or voice. You can recognize this signature if you succeed at a Spellcraft check when later identifying a spell to determine whether or not that spell was cast by the same individual. The spell greater magic aura can obfuscate this information, making it seem that someone else cast the spell. Greater detect magic grants a saving throw against magic aura (but not greater magic aura).

So the order of actions is:

• You cast greater detect magic
• You spend 1 round focusing on a specific spell's aura
• You make an opposed Knowledge (arcana) check against that spell's caster
• You later observe the caster in person while he/she is casting a different spell
• You make a Spellcraft check to identify the spell he/she is casting

If you succeed at this Spellcraft check and you succeeded at the opposed Knowledge (arcana) check earlier, you can recognize that both spells came from the same person.

Answer 2

You use the rules that are detailed in the spell

No, Knowledge (Arcana) would not be the appropriate skill to use to identify a spell. Spellcraft is the skill for identifying spells, whether it's as they are cast, or while they are already in effect. Knowledge (Arcana) is used to identify constructs, dragons, and magical beasts, or to identify runes or magical traditions.

When you use Greater Detect Magic to identify a spell, the method you use is the method that Greater Detect Magic tells you to use, not any other method.