ah maybe i worded it out bit wrong but i didnt mean that on succesfull identifying you would see the effects. i ment more like a text telling you what you have learned,vague text at best
Knowledge does not impart the ability to see the unseen
Ethinos said it well, spellcraft should have nothing to do in detecting the magic on someone. Detecting magic on someone propably enters the realm of someform of psychic talents, premonitional powers or so. Spider sense anyone? becouse how do you exactly are going to determine that someone has casted a spell on themselfs? spellcraft is out of the question becouse its more of knowledge of spells and magic in whole, its not a radar.
magic does not leave a trail one can sense, some monster may have some senses developed that are reacting to magic but as far as i know none of the playable races have such traits. so it would be silly to make magic detectable by good roll on spellcraft
so imo new spell for detecting magic is the way to go. it could be modified see invis spell or so, but scripted so that when casted, those who have some spell on them would glow for a very little duration on some color (could use the glow from ghostvisage or ethereal visage). and the glow would only be visible for the caster.
heck you could prolly even add that to true seeing spell, so you wouldnt need to create a new one