Nebdingo is spot on.
And if you are not confident checking a car for yourself the AA/RAC check is a good idea as well.
Another tip is take a mate and listen to his opinion, he won't have the same rose tinted spec's as you
Don't forget the HIP and compare it to the numbers on the car, the RAC own HIP and if you go to the RAC website they do the same checks through HIP but a lot cheaper.