you can take a fresh sample to a vet for a worm and cocci test..sometimes called a fecal float.
it will show what worms they have.
Wazine 17 (active ingrediant is piperazine..may be labeled Piperazine 17) is a good wormer, easy to use, and widely available, but only treats adult round worms (common in chickens)..
and should be repeated in 10-14 days.
sometimes you can see the worms in the droppings.
they might show loss of weight even tho they eat.
they might look "out of condition"
droppings can change color and consistency..such as foamy
egg production might drop
heavy infestations can be more serious and show more symptoms, such as weakness, (especially the legs), paleness from anemia, etc..
and lowers resistance to other illnesses.
here is a list and source of all wormers that can be used..they are broader wormers that treat more kinds of worms.
http://www.firststatevetsupply.com/store/index.php?cPath=23&osCsid=61f778551f090451e7dd6770345e3657
chickens get worms from the soil, bugs, earthworms, etc..
I worm my chickens twice a year...spring and end of summer or early fall before it gets cold. I try to avoid worming them when temps are extreme.