To check for lice, check each girl at the base of their feather & along the shaft. This is what I have in my notes for Lice
Ivermectin, 5 drops/hen, 6 drops/Roo or lrg, 3 drops/banty, repeat 10 days then 4 - 6 months, 8wk withdrawal (Depluming Lice - 1oz liquid soap, 2oz sulphur, 1 gal warm water, submerge chicken, spray coop/roosts/nest, remove all shavings)
For mites check under the feathering especially the vent/tail & under the wing area. Run your hand along the roost, you'll feel them crawling on your hand/arm. Permethrin, Poultry Dust, Sevin ... I dust my girls monthly with Poultry Dust rotating with Sevin.
You may want to post/search this in the Predator and Pest forum.