You unfortunately can not diagnose Marek’s Disease (MDV) online or by symptoms alone. There is also another disease that can mimic Marek’s Disease and it is called Avian Leukosis Virus (ALV). Both ALV and MDV symptoms can act out the exact same. Best way to know would be to get testing done on a bird that has passed away from the conditions. If it turns out to be MDV over ALV, then I suggest vaccinating any new birds that you choose to get. Unfortunately ALV does not have a vaccine available. Birds infected with ALV can ‘recover’ but be lifetime carriers of the virus, easily spreading it to a non-infected bird. The ALV virus can also be passed from the infected parents to the offspring, making any chicks that hatch, already infected with the virus, causing stunted growth, failure to thrive and painful internal tumors to form.
You can get testing done at your government extension, or if you’re in the US, you can contact your State of Agriculture veterinarian and ask about getting testing done on a deceased bird. There are also online laboratories that you can order testing kits from such as Research Associates Laboratory (RAL) or Zoologix.