Visual inspections would be to look for . . .
Discoloration in material/paint, warping of a flat surface, buckling, smoke stains, excess fluid/grease in wells/low areas, oil/lubricant burns (burned oil will leave a yellowish brown residue), grass or other debris that sticks to a flat surface where it should have fallen, excessive carbon build up or scoring, excessive weathering for internal components (for example thermal liner that indicates solar/UV exposure b/c of break down), water in what should be sealed assemblies (my fav where headlights & turn signals- "See Private, that turn signal is running low on blinker fluid! Go get some from Maintenance pronto, we do not want it to dry out!"), gray rubber seals (dried out, but not cracking- might be saved), cracked rubber seals, exposed wiring, flaking paint (why is it flaking?), rings in the paint around mounts/bolts, and . . . that is about it for what I remember for now.
You will also be checking for any difficulty opening hatches/access ports or unsteady brackets/mounts