Worming your fowl can be done as a preventative measure, or only when your birds show symptoms. If you keep your fowl on the ground, a preventative program can help you keep worms from becoming a problem, and can keep your whole flock healthier. To use as a preventative measure, worm your whole flock twice a year.
Symptoms
Diarrhea with traces of blood in it
Weight loss, especially around the breast area
Excessive food consumption
Weak, doesn't want to stand much or always sitting on the ground.
Lethargic
Treatments
Ivomec
Piperazine
Levamisole
Wazine
Fenbendazole
Warning
Do not consume eggs for a minimum of 1 to 2 weeks after the end of treatment for worms. 
Information above from Ultimate fowl
With that being said I do worm every 4 months with Ivomec (injectable) at the rate of 5 to 7 drops orally for adult size birds 3 to 5 drops orally for bantam size birds.  
Ivomec treats for worms and external parasites.
Chris