I don't have any advice particular to the Jem, but as with any guitar, take your time! sometimes I'll get excited with the guitar, and forget to check for details. You should:
-Check for cracks: specially in the neck pocket and headstock. Neck joint cracks are NOT always only superficial. See 'All About Evo'!
-Check neck straightness: you don't need straight edges or something, just look carefully pointing the neck towards light.
-Check EVERY fret, on EVERY string. Play every note on the fretboard and see if there's buzzing.
-Play with the trem: it must have stable tuning (if not, you can fix it by reading here or Ibanez Rules). ANY weird noise is BAD noise.
-Take your screwdriver, remove plates and look for bad wiring and stuff.
-Ask questions. you are spending your money, so you're entitled to!
Hope that helps!