The gauge/chamber length will be stamped on the barrel just ahead of the barrel extension. It will be an oval with either 16-65 or 16-69? inside it.
Location of the safety will also tell chamber length if short chambered. The pre-war guns had 2 9/16" with the safety ahead of the trigger.
bangeye mentioned choke: The older guns were choked for use with fiber wads and shot in contact with the barrel wall. They have more constriction than modern barrels.
