Crawling underneath the car...
Crawling underneath the car to look at the oil pan and center linkage will tell you a lot about how well the car has been maintained.
It all comes down to how well the car is maintained. If the car has a lot of grease and grime throughout and simply looks like it has had a rough life, then it probably has. One of the most overlooked areas of the car is the center section of the chassis. This tells more about the chassis than the front and rear clips ever will. A chassis typically has sheetmetal welded throughout it. Examine the cross member bars that run along the floorpan of the car; if any weld is broken and you can see a hole between the chassis and the sheetmetal, then somewhere the center section of the car is tweaked. If you see anything like this, make sure you have the car examined by the chassis manufacturer if at all possible.
Ideally, you should have the chassis manufacturer put the car on a jig. Or if it's a street car converted to a Street Stock or the like, take it to a body shop employing a frame expert. This is to determine whether anything is tweaked and to make sure the seller isn't trying to pull a fast one on you. If they are unwilling to work with you and schedule a time for the car to go on the jig, then something is up and you shouldn't purchase from them. If you do get it to the manufacturer, while you are there, ask the manufacturer how much a brand new, race-ready car costs. This will serve as a reference point for how much you should be willing to pay for the used car. If the asking price were only $2,000 or so less than a brand new car, then it would be unwise to purchase the used car.
If possible, your best bet...
If possible, your best bet is to get the chassis manufacturer to put the chassis on the jig to make sure everything lines up, and that the chassis is not bent or warped in any way.
The engine is another beast entirely. Make sure you ask how long it has been since the latest engine rebuild, and who performed the rebuild. If it is someone other than the owner, call or visit the person and find out as much as you can about the engine. If the car owner was the last person to rebuild the engine, then make sure you have an engine specialist who could spend an hour or two inspecting the engine and making sure it is up to par.
Another good indication on how well the engine has been maintained is the oil pan. When an engine blows, it will often blow a hole in the bottom of the oil pan. Crawl underneath the car and look to see if the oil pan has been welded back together. While you're underneath the car take a look at all of the front suspension components and look for any hairline cracks in the center linkage. This will show you if the center linkage has drug the track because of an accident. You can see how well the car has been maintained by just looking at everything from underneath the car.
If the truck arms have any...
If the truck arms have any bends, then you can count on the rear clip being bent.
If you follow one piece of helpful information, then do not purchase the seat that comes with the car. You need to spend the money on a seat that is custom made for the driver. Remember, this piece of equipment is protecting the most important thing in the car-you. I recommend a manufacturer with a reputation for quality, such as ButlerBuilt, The Joie of Seating, or ISP.
Ask The Tough Questions
I cannot stress how important it is not to rush into buying a car. I suggest arranging a visit while the car is at the shop, and not when the car owner is at the track. This will give you more time for a one-on-one discussion about the car. You might have to ask some difficult questions, but they must be asked. One of the most important questions you can ask, and one that you more than likely won't get an honest answer for (funny how those always coincide), is "How much did you pay for the car?" If they built the car from the ground up, they might not know exactly. If they bought it used, then they should know. If they paid $20,000 for the car and are asking $20,000 after two full seasons of racing, then an alarm should be going off in your head.