But seriously - suppose I was a seller and wanted to prove with pictures my frame/fork weren't bent, how would I do that?
A good question.
In my case, I eyeball the fork from both sides and square on from the front. A center line, drawn through the head tube and extended down the fork blade will help to determine bent or unbent. Hard to explain how to do this but easy enough to do. If the center line of the head tube extends down the center line of the fork, you are OK for that part of the gauging.
Looking directly square on at the front, does the wheel evenly split the distance between fork blades? If you, that is another positive suggesting no fork damage. If the wheel does not evenly split the distance, the problem could be either a bent fork or an out of true wheel (just turn the wheel around and install to see if the even split thing changes - if not, the wheel is probably OK...
Cracks in the paint are an immediate indicator of frame or fork trauma. When you see these cracks, you better know what you are doing to make them go away and get everything true and good to go.
Riding the bike will also help to indicate a bent something or other. If the bike pulls to one side or the other, suspect head set issues or a bent frame or fork.
Other than that, just ask us and we will all tell you what we think. In my case, I suggest a problem only if I really see something that raises red flags. Often times, it is picture distortion. Those other times - pooey stinko!