Coraline's Spell of Schematics.

By forming geometric arcane sigils with your hands and using the shapes to look through the caster can identify a spell allowing the caster to know what spell was casted and the schematics or blueprints for it. 

This can be done on a spell that is active on a person or object or it can be done within 48 hours after the spell in question has been dispelled. 

Just because you know the spell schematics afterwards and you know the original casters signature, does not mean you are capable of casting it.