If you think your ferret might have a cryptosporidium infection, the best thing to do is to go to your vet. They can do a full physical examination, and check your ferret over from nose to tail, especially as diarrhoea can have many causes!
To diagnose cryptosporidium your vet may recommend faecal tests. These will look for the cryptosporidium themselves, which are invisible to the human eye, and require a special test to see under the microscope. These tests are often part of finding out why a ferret has diarrhoea, and may also look for common bacteria and other parasites too.
If your ferret does have cryptosporidium, your vet will create a management plan to support your ferret’s recovery. Most healthy animals can overcome a cryptosporidium infection in several weeks, but in ferrets with a poor immune system this could take several months.