Airframe overstress may not give any signs that would be picked up on a simple visual inspection. One reason why pilots need to report events that may have led to an overstress.
There's a few obvious signs of serious overstress, but you've had them during PPL training. Buckling/denting/cracking of skin is the most important one. For landing mishaps, check the welds on the engine/nosewheel mounting frame, if you can.
On our aerobatic R2160 we perform one additional check. Squat down about two meters behind the aircraft, exactly on the centerline. Look at the place where the top line of the elevator intersects the wings. This should be completely symmetrical. If it's not, somebody bent the plane, typically through torsional loads due to side loads on the rudder (eg. flick roll at too high a speed) or through the wing root bending effect: rolling while pulling serious g's, which increases the g loading of the wing going up, but is not reflected on the g meter in the panel.
I don't know of any other checks that you can use to visually check for overstress. It's very insiduous, as mentioned before.