I think you should be able to see the mites if you look closely. Perhaps check around the base of their bills, their eyelids, under their wings.
But I would check their shelter - what do you use for bedding? You may need to do a big clean out of the bedding, and then sprinkle DE in every place where wall meets floor, cracks in boards, as that's where I have read that mites hide.
There are products you can use, but please be cautious. Do the ducks seem healthy otherwise?
There were some mites (they were tan colored) in a bale of straw I had in the night pen. ick. I tossed that, far away, pulled out the bedding, wiped the floors with neem oil solution, put the DE in all the cracks and crevices, and I think we dodged that one.
Do your ducks swim daily?
They seem perfectly healthy otherwise.