Just based on my experience... anybody walking around a gunshow with one on their shoulder is selling it. The one time I bought a gun at a gunshow, I had the seller stick it under the table so I could pick it up on my way out the door so I didn't have to carry it around.. I'm assuming I'm like everybody else, and I can't see why anybody would want to hump a 26" double-barrel around on their shoulder for 3 hours while they did the rest of their shopping.
Of course this is a moot point in states with waiting periods, but in the normal states, I think it's 99% safe to assume a guy with a gun on his shoulder is selling it.
And while I understand there's a certain amount of "etiquette" involved, I often see dealers buy guns off guys carrying them around in the crowd... business is business.