It may be old advice, but every waterfowler should pattern his or her shotgun before opening day. It's a good idea to shoot your gun at different ranges with various choke tubes and several different waterfowl loads. If nothing else, patterning your gun can reassure you that it shoots where you point it.
Ron Walker, inventor of Wad Wizard choke tubes, spends a significant amount of time patterning shotguns. "When patterning a waterfowl gun, I like to have a target the size of the bird I'm hunting with about three feet of cardboard backstop on each side of it," Walker says. "I don't shoot from a bench; I just stand up, mount my gun, and shoot at the target as if I were hunting."
What you're looking for is an even pattern with a sufficient number of hits on the target. Some authorities say five pellet strikes with No. 2 steel pellets are ideal, but the goal is an evenly distributed pattern with no large gaps.