If you're worried about it best thing would be to take it to a good garage. What state are you in? I know here in Victoria, Mick at Pitlane has quite a few different types of pre-purchase car inspections. IMHO it's well worth the initial investment as not only can you avoid a thrashed car but you may be able to bargain with the bloke on price if you have a list of faults. Things like the ZF auto milk shaking or a car being put back to stock aren't always visible with the naked eye.