Ridley's Cohort said:One could simply assume that spells include subtle observable magical manifestations during casting that hint at the nature of the spell. Someone with a high Spellcraft skill would know how to interpret these signals.
I've never seen the least mention in any book that that is the case. Not even Tome & Blood mentions a spell's "magical manifestation during casting." On the other hand, the Psionic Handbook explicitly describes the manifestation of powers. Your eyes don't crackle with electricity while casting a lightning bolt spell, for example. You just speak the words and make the gestures and after you're done a bolt of lightning appears. If there were more to it than that I assume it would be mentioned somewhere.