If you want to learn how to palpate for bladder stones, your vet should teach you how to it. But, I'm pretty sure your vet will tell you that twice yearly checkups are enough to keep any new stones from growing dangerously large before detection.
Did he discuss any methods to diminish the possibility of new stones? like depending solely on fresh natural food for nutrition, without feeding any supplements, especially mineral supplements? Iguanas are designed to eat large quantities of high fiber low nutrient greens, flowers, and occasional fruit. It's strongly suspected that supplements cause medical problems like bladder stones and possibly shorten life.