Bob has referenced the AMA requirements correctly. Doc and Joe are referring to the "old time" procedures, requiring AMA numbers on the upper right wing at least 1" high. I still do this, too, on most airplanes, although it is no longer required. For my Corsair, in order not to disturb the scale-like markings, I wrote the AMA number on the firewall, clearly visible by looking into the cowl from the front. This clearly meets the present rules. I figure if the contestant can show you the AMA numbers and you can read them, the airplane is legal. If he says the AMA numbers are inside, not visible unless you cut the airplane apart, I would ask him to show me...
Pete