Things I'd look for are:
- Approximate round count, incl. minor or major loads
- Prior usage (USPSA, Steel Challenge, etc.)
- Overall condition, how does it look (carbon settings, breach face, comp, etc.)
- Does everything work flawlessly (safety, mag release)
- Fit of slide to frame (not too much movement)
- Status of the frame and slide to look for any cracks
- Fit of comp on barrel (alligned, straight, no movement)
- Status of barrel rifling