I usually worm my chickens every 1-3 months, or more often if needed, and my peacocks every month! Especially if it is either rainy or cold outside, they are more succeptable.
You can tell if a chicken needs worming simply because it is getting thin; the breast bone is more prominant. If it is really bad, the bird will become "skin and bones", pale, and extremely weak because the life is literally being sucked out of it! They will usually continue to eat until they get to this point, and hopefully yours are not! (If they are, just let us know and we can try to help you with it.)
Here is how you can worm them, if they are feeling under the weather or just as a preventative: I use 10cc of Ivermectin Sheep Drench per gal of water. Leave them on it for two or three days and your good! Since it is expensive, if you have a small flock, just see if you can pay a friend to take one or two doses of it.
You also need to look out for Coccidiosis. The birds get skinny and often show signs of blood in their stool, if it is bad enough. (this is often a common cause of chick death...usually traces of blood is seen in thier stool). For this, add 2tbsp of Sulmet to one gal of water, you can use the same water as the Ivermectin. This is only about $8 per bottle and you can pick it up at your local feed store.
Good luck and let us know if you need any more info!
Crystalchik