Some of the attributes of a pigeon gun will be as follows. It will be something on the order of a pound heavier than a game gun, say 7 1/2 to 8 pounds. Balance (teeter-totter point) will be about the same to maybe a 1/4" farther forward. Swing will require something like 20-30% more effort than a game gun. The action will be strong enough for 2 3/4 - 3" hulls and 1 1/4 - 1 1/2 oz loads, significantly more powerful than game loads. Stock drop tends to be a bit less than a game gun as a pigeon tends to be a rising target, but is not absolutely so as in trap clays. Lack of a safety is a strong indicator. Many pigeon guns were built to high original quality standards. Many will depict a pigeon somewhere on the gun and some are marked as pigeon guns. Purpose built pigeon guns were in both hammer and hammerless.
Modern guns of better finish are sometimes marked as "pigeon grade."