To determine if you have mites, depluming mites or a feather picker, you need to assess the damaged feathers.
Broken feathers can point to a feather picker. If the skin is red and irritated, coop mites could be your problem. To check for coop mites, go out after dark and wipe a damp white cloth over the roosts. If it shows red smears, you have coop mites.
To determine of you have depluming mites, look carefully at dropped feathers. If they are very thin and threadbare, like a pillow case washed over and over for too many years, your chickens could have depluming mites.