First, examine a few chickens that appear to be scratching. While mites may not be readily apparent, you will easily see lice crawling around the vent area. To determine mites, look closely at the skin for redness and scabs from bites. Acute mite infestations cause anemia, so if combs are paler than they ought to be, that points to the possibility of mites. To confirm mites, run a damp paper towel over the roosts at night. You should see red smears if mites are present.
Spraying for mites and lice doesn't mean drenching the chicken. You can choose a sunny day with no wind and spray the skin around the vents and under the winds and around the neck, avoiding feathers as much as possible. Let the chickens sun dry or blow dry them unless you have an impossible number.
You will need to spray the coop if mites are present. Again, choose a period where the weather isn't going to be in a deep freeze so the coop can dry out and not present a damp condition that promotes frostbite. It's pricey but very effective. No need to spray the coop for just lice as lice live their life cycle on the chickens.