Many times the state vet can recognize signs during the necropsy, such as tumors on organs, enlarged nerves in the legs, eye changes, or skin lesions. If they suspect it and the owner wants a PCR test, they test the feather shafts and any tumor tissue they find. Many states send off the Mareks test and it is an extra charge. Mareks is a worldwide disease. I have never seen it in my flock, even though I have some vaccinated and some that are not vaccinated birds.