A bit time consuming but the most accurate way is to pattern the gun. Put a very large pice of paper or cardboard targer at 40 yards from where you shoot. Shoot each barrel once at a separate target, find the largest mass of holes, draw a 30 inch circle around them. Count the holes in each target. divide the number of holes by the number of pellets in the load selected and determine the percentage of holes in the 30" circle. The percentage will tell you what you are shooting for choke.
Or less accurate but less time consuming, find a gun store or friend with a choke gauge and measure it.
