Take a fresh fecal sample to your vet and have them look for worms is the best way.
Another sign of worms is skinnier chickens and ratty feathers, as well as worm segments or whole worms in the poop or in eggs. That would be an advanced infestation.
Or, you can worm on a preventative schedule and do it once or twice a year. If you do this, try to rotate active ingredients so the parasites don't become acclimated to any one active ingredient. We worm twice a year, using Ivomec during the fall moult (there aren't so many eggs to throw away) and Eprinex during the spring moult (no egg withholding). We worm our dogs and horses and goats on a schedule, so we put the hens on a schedule, too.