I've been shooting shotguns forever, and I had no idea there was a special tool for the job. Most tubes and barrels say what the choke is somewhere on the surface. Be aware, however, as many as half of the chokes I have actually checked by patterning did not provide the indicated result. IOW, I've seen mod. chokes put less than half the pellets into the 30 in. circle at 40 yds. I've also seen imp. chokes put more than 60 % of the pellets in the circle. There's only one way to find out what a choke TRULY is with a given load... Throw some lead.