Well, there are different types of mites...
Fowl mites and feather mites are both visible to the naked eye and easiest to see at night. They're little bugs crawling on the chicken. Feather mites will also lay eggs on the feather shafts and look like miniature rice stuck to the feather.
Scaley leg mites are not visible- but always end up "raising" the scales on the legs. If the leg scales are not smooth...then they have them.
If one has them...most likely all do...so treat them all.
Fowl and feather mites. I personally use FOOD GRADE diatomaceous earth. I "dust" the chickens liberally and the spread it around the coop, nest boxes, and throughout their favorite dust bathing areas. I continue it's use in the coop, boxes, and dusting areas as a preventative. It's a 100% natural way to deal with it. If you have a hard time finding it right away, then you can use poultrydust (available at most feed stores). The last option is sevin dust- available at garden stores. Each option is a little more toxic...but all are effective.
Leg mites....best treated with any kind of oil and/or vaseline. Some say kerosene works well but this is extremely harsh on their skin as well as their lungs and I DO NOT recommend it. My prefered treatment is mineral oil with some tea tree oil mixed in. The tea tree oil has both antibacterial and antibiotic properties so it's good for any sores they've developed as well. Cover the legs well...do not wipe off. If you have a lot of chickens...put the oil in an old coffee can or the like...and just dip their legs into it. Feather legged chickens are "uglier" to treat because there's no real way to treat the mites without getting their leg feathers all oily and then the dirt will cake on them...
I also use orange oil on all the wood in my coop---especially the roosts. I use "orange Guard" available in the pesticide section of Ace Hardware stores...although it's 100% organic.
GOod luck
Sandra
OH...I forgot to say that if your chickens have feather or fowl mites...you might end up having some crawling on your arms after you handle them. Although it feels absolutely gross...do know that these mites are bird specific and won't harm you. Personally...I think they're on me until I take a shower! ewwwwww!