Re: 1990 GT
If you look at a manual, make sure the clutch pedal releases easy, fairly early, and without chattering/shuddering
Pop the hood/look around under the car check for leaks/rust spots (esp. the front floor pans, I had one in my 89 rot out above the brake lines on the passenger side...)
Check for an unusual amount of rubber buildup in the rear fenderwells, it can be a sign of how frequently the car was abused..
Take it for a test drive before you buy, don't just look it over then take it.. be sure to drive it, get on the gas a bit, make sure the shifts are good (if auto) or synchros don't grind (if manual) and be sure to look behind you if you gun it a bit, see if there's a plume of smoke following you, that's usually a bad sign..
2012 IS F Blue IS F Sport
2006 Mustang V6 wannabe