Do you have a piece of WHITE sheet plastic, like real estate sign plastic, that you can smear with Crisco shortening, or vaseline? Then slide it carefully and slowly onto the floor of the bottom board, leave for a 3-4 days, then carefully pull it out and count how many mites you have on it.
If you have a screen bottom board, just slide the plasic board underneath it.
Do it in the late evening, or just dark when all the bees are in and settling down.
You can also open up some drone brood and check for mites.
If you find mites either way, Formic Acid Quik Strips work wonders on killing all of the mites even in the capped brood. They are really easy to use as well. Lots of Youtube stuff on Formic Acid Quik Strips.