Pattern it with several different loads. Look at 20, 25 and 30 yards. I like tight chokes with decent but not top end speed. I seem to have better luck shooting 1/2 ounce loads with velocities between 1250 and 1300. If I can get a 18-20" killing pattern at 33 yards I am as happy as I can be. I have found by limiting my shots under 33 yards my success rate climbs. And by 33 yards I measure out 33 yards and place markers, as simple as cut weeds to establish known distance markers to judge birds and leads. With a Full choke and 1/2 ounce loads I have taken many an early season limit. Later I go to 5/8 or Stan's favorite 3/4 ounce loads.
But choke gauges tell you almost nothing about your gun. For that you need to shoot it at a pattern plate, multiple times, under game type distances. 40 yards is worthless distance for a .410 and 21 yards, a common Skeet distance, does not give you a realistic hunting situation in too many cases. 20, 25 and 30 will give you real game type information about how your gun will work. You might find your current set up works out to 30 yards with no changes needed or you might find the point of impact is high or low at that distance. Shooting will tell.
