Keep the liquid, will be very handy if you find they are roost mites rather than poultry mites.
Here's some tips on how to examine and identify:
Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.
Part the feathers right down to the skin around vent, head/neck and under wings.
Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.
Look
fast, they will scatter quickly once the feathers are parted and the light hits them.
Wipe a white paper towel along the underside of roost to look for red smears(smashed well fed mites).
Good post about mite ID by Lady McCamley:
https://www.backyardchickens.com/threads/my-chicken-has-mites-now-what.1273674/page-2#post-20483008