MRI. There are characteristic MRI findings. Certain test can be done on spinal fluid too.
Immune suppression. The jc virus is present in at least 60% of normal individuals and truly does little thruout our lifetimes. Pts with HIV infections, or individuals being treated with cancer chemotherapies, or other immune-suppressants are also at risk. Tysabri (natalizumab) for ms is also a provocateur. Testing of blood or spinal fluid can be confirmatory, and a brain MRI can be characteristic.