If you think your reptile 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 reptile over from nose to tail, especially as diarrhoea can have many causes!
A major difficulty with cryptosporidium is that it can be very difficult to diagnose. Often cryptosporidium is not picked up by faecal testing under a microscope and more complex tests looking for parasite DNA need to be done. Even this is not 100%, as the test will only work if the parasites are in the faeces to be found. As cryptosporidium can hide in the body, and may only be found in the faeces intermittently, if the parasite is not currently there a ‘false negative’ result can be achieved. The only certain way to diagnose the parasite is on post mortem microscopic examination, which is by no means ideal, although more sophisticated testing methods are being developed.
A second difficulty with cryptosporidium is that there are no treatments that are 100% effective. Sadly, many affected reptiles are already very unwell by the time they are at the vets, and may be past help. The best management plan for your reptile will be developed by your vet and you after diagnosis.