If there is no obvious damage or cracks in the frame. Check for wear and tear. slop in the rear derailleur, marks inside the front derailleur, the shape and width of the teeth ,chain wheel and cluster (have a look at a newy to see what they should look like).
and look at the wheels where the spokes touch each other. heavy use will show as an obvious divot.

If all this checks out then you are probably on to a good buy

A fit up and service wouldn't go astray.