First test is mites on bird during day is NFM. Mites on roosts and birds at night RRM.
RRM cling to corners and roosts, so a flashlight at night will show them. They are clumps of black and red (black unfed, red fed).
NFM are black specks crawling like crazy during the day, usually at the vent, but other areas as well. A few can fall into the bedding in the coops, but generally they stay and live on the bird. NFM die without close contact with the bird.
On the other hand, RRM can live for MONTHS in the coop corners. They are very hard to kill and the old timers at times resorted to burning the coop down and starting again to get rid of an infestation.
Both are tiny, but can be seen by the naked eye.
I'll link photos for you.
I've had NFM, but thankfully, knock on wood, not RRM.
LofMc
https://the-chicken-chick.com/poultry-lice-and-mites-identification/
https://www.flytesofancy.co.uk/chickenhouses/Red_Mite_in_Chicken_Houses.html
http://www.poultrydvm.com/condition/northern-fowl-mites