A used car can be a bargain or a very expensive mistake, and the difference is usually visible in twenty minutes if you know where to look.
Start with the papers
Ask for the RC, insurance and the service book before you look at the car itself. If the chassis number on the RC does not match the one stamped on the car, walk away.
Then the body
Look down the side of the car in daylight. Ripples in the reflection mean panel work. Check that the gaps between panels are even on both sides.
Finally, the drive
Drive it cold. A car that has been warmed up before you arrive can hide a lot.