You can take in some fresh droppings to your vet to test for what worms and worm eggs your chickens may have. Tapeworms may not effect all of your chickens, so you only have to treat the ones who have them visibly in their poops. With other worms, most of the chickens will have them if one has them.
Do you have any pictures of the worms or the poop? Tapeworms are flat, and usually you will see just the segments. A product with praziquantel is the best treatment for tapeworms. Equimax horse paste is probably the cheapest, but it is also in Zimectrin Gold horse paste, and Droncit tablets...