Each chicken owner does their own thing regarding "de-worming" or "worming". When I showed my birds I "dewormed" them once a year when they were in molt. I used SafeGuard horse paste. A glob the size of a pea over the tongue. Now, I don't show, so the Girls don't get treated at all. They will if I see worms or if they become unthrifty and there has been no change in feed, they are not stressed, and if the weather is not a factor. I also "deworm" and dust any new flock members while they are in quarentine and any birds I sell.
Basically, treat if you see worms or have a good suspition. If not, then don't treat. Over treating for parasites can cause problems later on down the road as the parasites develop a tolerance to the drugs in the medications. The horse world is starting to go through this now.
If your suspicious of worms, technically the correct thing to do is to take a dropping sample to your vet and have them test it to determine what worms (or lack thereof) are present and "deworm" according to what type of parasite is found.
Honestly I never bothered with all that when I showed. I just did it once a year and never had issues. The vet in our area at the time was very expensive and knew nothing about chickens.
Generally speaking from my experience, a "wormy" chicken will be unthrifty and pale faced and can have poor feather quality because the nutrition form their feed is not being utilized correctly. They usually will behave normally. Once "wormed" the chicken will start to gain weight and their face becomes normal colored again. Their face gets color pretty quickly. HTH