What do Pilots look for

While sitting on my delayed AA flight this afternoon I took particular interest at the goings on next door. At the gate next to mine at DFW the pilot was taking his time to meticulously review the outside of the plane. Wings, wheels, panels. It left me thinking. What is he looking for? I mean beyond the obvious- wear and tear, cracks, missing pieces? Are pilots trained to do this? Pilots please reply.