To identify crypto take at least 4 samples of freshly scouring calves to your veterinarian.
There are rapid test kits that identify the presence of Cryptosporidium parvum. Traditional laboratory tests indicate the number of cryptosporidial oocysts and is correlated with the severity of the infection. The latter test is the preferred test as it can confirm the scour is associated with cryptosporidial infections. It is however more expensive and requires the samples to be sent away. Any diagnostic tests should also show the absence of other scour causing problems such as rotavirus, coronavirus, E coli, salmonella and possibly one of the less frequent causes of scours such as, bovine virus diarrhoea and coccidiosis. The results always need to be interpreted and other factors taken into account such as age of the calf when scouring. Post weaning coccidiosis is underdiagnosed and has a real economic impact without signs necessarily being present.
All these tests to exclude other problems can add further costs but are necessary to aid in improving the reliability of the results.
While you are waiting for a diagnosis, start electrolyte treatment with Kryptade (sachets are ideal for this purpose) until you have a confirmed diagnosis. This will provide you with the most cost effective treatment especially if cryptosporidiosis is involved.