Check the floor pans, shock tower (under the battery), and the lip of the hatch for rust..
Check torque boxes, make sure they aren't distorted/ripped. Check for cracks in the floor pan. The bolts for the seats like to rip out as well...
Pull the cowl cover off and check for rust in there, also make sure the cowl sheet metal isn't full of filler (from the hood flying up.)
Look for any signs of previous wrecks. And run a car fax!
Make sure the heater core isn't by-passed and run it to make sure it works (the heater core can be a pain to change.)
Check all the light. headlights (hi-low), taillights, brake lights, turn signals, etc.
Run the car long enough to make sure it doesn't over heat.
If it's a 5 speed, run it through the gears at a higher rpm and shift fast to check for grinding.
Check ALL the fluids and look for anything abnormal (water in the oil, burnt tranny fluid, etc.)
YOU (and the owner) take the car in for a safety inspection before any transaction are made.
That should give you a good idea of what you're buying.