How to know if i have pinched nerve or piriformis syndrome?

Pyriformis/nerve: Its very difficult for you to tell. But a doctor who does a proper examination should be able to tell. Sometimes its difficult without an injection being done.

Piriformis syndrome: The sciatic nerve or one of its component when pinched can cause "sciatica". At the spine level it would be one of its components but once outside, it can be pinshed by the piriformis muscle causing similar symptoms with added pain from the muscle itself. A nerve conduction test preformed by a neurologist or a PM & R specialist can diagnose it.