I go out at night periodically and check their vent area with a flashlight. Doesn't make the girls happy, but only takes a second. The mites come out at night and will scurry with the light from the flashlight. They are brownish and small, but you'll see the movement.
To dust the chickens, I like Miss Prissy's pantyhose method the best -- fill a pantyhose foot with pyrethrin, turn the chicken over in your lap and poof poof poof. Use your fingers to ruffle their feathers so the dust goes to the skin level. Make sure you get under their tail and under the wings. Takes less than a minute per chicken and you're done. I do it in the coop so any residual dust goes into the shavings. Not a bad idea to dust the roost area and the nest box too.
Red mites are a real problem here when it's cold -- you wouldn't think they could live when it's subzero, but they can. Grrrrr.
Hope this helps.
CC