Well, you got to watch out for the "Texas Blend" which takes up the whole side. Something you learn about in Wyotech.
Also do a "knock" check for body filler on any area that seems "uneven" or the paint doesn't look as perfect as other areas.
Also, be sure to look for body damage, even if the car "looks" okay. Look at the suspension, look at bolts on exhaust or a cut heat shield and things of the sort.Also, if bolts in the same area to the same thing (like header bolts or exhaust bolts). There shouldn't be a stainless steel bolt any where on anything like that, nor 2 different "colors".
If the engine bay looks all gussied up from the bottom and the top, get a flashlight and look closer to the block and look for any new leaks. If there's a smell, make a note of it. Check the harnesses if you can, some modders remove and replace things that will "bypass" things such as an EGT sensor that will more than likely be unplugged and have a resistor plugged into the harness end. Things like this are dead giveaways to walk away from the deal.
These are all things I look for now when checking cars out since they are things I ran into after buying my bugeye or after test driving and doing a walk around w/ a very close eye. NEVER BUY A CAR AT NIGHT!
There are plenty of other things to look for, sometimes, if your intuition tells you there's something wrong, there is.