The main sign is related to
the gaps between body panels
If they are uneven or too large, this may be the result of poor repair after a serious accident. this is especially true for cheaper used cars that are not repaired in professional workshops, but in "garages" such.
Experts also advise to take a closer look at defects on the body, especially if there are any on the thresholds. A possible more serious previous impact there may have disturbed the geometry of the car.
The buyer should also be aware of the uneven coloring of the bodywork.
Repaint marks indicate that the car has been involved in an accident. He may have been light, but he may not have been. It is good to check the presence of putty under the paint, as this is easiest for non-professionals to do with a magnet.
The thick putty coating does not attract the magnet
this is almost a sure sign of poor repair after a more serious impact. However, this is provided that the fenders and other body panels are metal.
Steering wheel position
This must also be monitored, because it is a prerequisite for the impossibility of adjusting the car. Check with a test that the car is moving straight. If "withdraw" in any direction, this is a sign of a violation of the geometry, a consequence of a previous impact.