no absolute way. But look at the signs. lesions.. mostly about the head. bloating and dropsey. Wasteing away. Bad fin rot that moves very quick and into the flesh. If you look down the sides of your fish and see odd patches with a little fuzz look to it. ALL are symptoms.. treat for the obvious and if it fails consider the mycos. Or send to a lab. It took a lot to get the final diagnosis of the strain. And it was done for the number of fish I had. My local vet wanted $900 to work up 3 fish. I had 30 tested first, then another 24 I think. I will send more this summer to confirm it is gone. Not everyone is going to get that from a research facility. So keep this in the back of you mind as always a possibility..How can you tell they have pathogenic M. without lab testing?