You could try to go ahead and worm with Valbazen or SafeGuard liquid goat wormer, or ask if you can take some fresh droppings in to your vet for a fecal float.
SafeGuard liquid goat wormer or horse paste dosage is 0.25 ml per pound of weight given orally for 5 consecutive days. If it is given once and again in 10 days it will only treat roundworms.
Worms are in soil, so chickens will pick up the eggs. It helps to keep them on grass, and let them free range. If they are confined to a coop and run, keep it as clean and dry as possible. There is a lot of misinformation about so called preventatives. When I joined BYC, a lot of people were saying that ACV, cayenne pepper, and pumpkins seeds would prevent worms. No scientific data to prove that. I tried those, but the best things are keeping your chickens in a fairly clean and healthy environment. Keep your bedding stirred, dry, and add to it as needed. Change it before you have an ammonia odor. Clean water daily, clean food, etc.