Look under the wings and around the vent. Look for straw colored bugs (lice) or black dots (mites). What happens for me is I get itchy after holding chickens or going in the coop. Or I see them shaking their heads every 5-15 seconds as if to get something off their necks. The mites DO bite humans but cannot breed off us, so you might find a bite on you.
I use Poultry dust (permethrin) from the feed store and permethrin spray (for the coop). I use a tied off sock to apply the dust everywhere except the face. Repeat in 7 days for mites and 14 days for lice (better results if you do it every 7 days -they will lay fewer eggs to hatch out). There is the red mite that lives in the coop and so spraying the coop cracks is important too. You can dilute the liquid permethrin and spray it on the bird also in nice weather (see under label for directions) - the dust is VERY messy.
Wild birds bring in mites and lice.
The poultry lice and northern fowl mite (and red mite) cannot breed on human blood. The NFM lives for 3 weeks with no poultry blood meal and the red mite lives for 9 months with no poultry blood meal. So if you collect them in your home you can vacuum them up or they will die eventually anyway...if you have a bird's nest outside the window and they come in by the thousands an exterminator may be needed.
http://www.thepoultrysite.com/articles/934/external-parasites-of-poultry
be sure to check pesticide approvals to keep current on what is approved for poultry
http://npic.orst.edu/factsheets/Permtech.pdf
I just found this sheet.
Diatomaceous earth, woodstove ashes, sand, and dirt can make a nice mixture to ward off mites. I put ashes in a metal pan for the chickens during stove season and they love it. Will turn the poo black as they ingest charcoal. Ashes are 50% calcium and very good for them- they love to eat them. DE will not cure an infestation.
Some have experimented with neem oil (there is a thread on BYC)- maybe one day I'll try it. I have repeatedly dealt with mites in my flock, and have been so miserable with mite bites- if you don't toss all shavings in the nest boxes and coop they come right back. I use sand in the coop now and feel that I have fewer mite problems. I use shavings in the nest boxes.
I now treat my flock every 2-4 months to keep mites away. How I know it is time to treat is when I get bitten or feel itchy. Then I treat the flock, retreat in 7 days and presto! GONE! (Also I spray the coop.) I throw dust around on that sand in the coop also.