A second-hand car can end up being a bad deal if you don't know what you're doing. Thankfully Popular Mechanics has put together a comprehensive checklist to help used car shoppers strike a good deal.
The checklist is a two-page PDF that you can download for free at the link below. The checklist marks questions in four colours: Green (not serious), yellow (cause for concern), red (serious issue) and blue (deal breaker). Once you run through the checklist, tally up your greens, yellows and reds. Compare the numbers with the scorecard at the end to figure out whether the car is worth your time and money.
Popular Mechanics has a few more tips on how to inspect the car, which might be worth reading.
Used Car Checklist (PDF) [Popular Mechanics]