After the Annual is done, I always get the maintenance firm to leave inspection covers off and I go around with a light and check everything is secure. Then it goes outdoors, I do the ACF50 treatment (a right messy job) and they put the covers back on.
Reversed ailerons should be obvious during the full and free controls check - they should move opposite to the yoke.
a Russian airliner, recently reassembled from cannibalised parts, in which the ailerons were installed wrong way round
It beggars belief that one would get that far without anybody checking. Maybe the pilots cannot see the control surfaces from the cockpit.