All right, let's try this the most direct possible way. Keltham internally contains a Read Magic cantrip (and a Guidance cantrip) and can feel the structure, which looks the same as the sketches in the textbook. That's useful for the visualization part. Keltham will then meditate and try to feel magic at his fingertips, like when he cast Resistance and Detect Magic before - he was paying attention - as he holds his hand over a copy of Read Magic built up over somebody else's spellbook. He will try doing things with any magic he thinks he might be feeling, and be told what, if anything, happened when he made a motion.
Possible hypotheses to distinguish include:
- Keltham, as a being of dath ilan untouched by gods, will prove to have zero magical aptitude and unable to affect the magic in any way. (Seems unlikely if he's a cleric and cleric spells look the same as wizard spells.)
- Keltham will have unworkably low wizard aptitude, as a result of coming from a society that has never selected on itself at all for facility with wizardry.
- Keltham, having not come from a society in which wizards have had more access to contraception for however many generations, will do great at this.
- Keltham, having the mighty mental disciplines of dath ilan at his disposal, and having played a fair number of subtle perceptual computer games, will do great at this for reasons having nothing to do with genetics.
- Keltham will make a perfectly normal amount of progress for a Golarionite cleric with 18 Intelligence and zero prior magical exposure.
- The first ten minutes of testing will not be enough to distinguish any of these hypotheses, because they're going to initially produce flat failure and that would've been true for any realistic sort of human being.