What is the consistency of a cancerous lump on the testicle? how can one be sure it is not part of the epididymis?

Cancer is hard lump: Cancerous lump feels much harder than testicle itself. Lumps in epididymis usually smooth & almost always due to a cyst. Hard lumps of epididymis & spermatic cord occur, but very rarely. Consult urologist when in doubt, scrotal ultrasound will provide answer. Head of epididymis lies on top of testis. Epididymal cysts can be tender to touch, cancerous lumps non-tender unless hemorrhage has occurred

Simply put: See a urologist. Tumor/cancer is usually hard and PAINLESS. Epididymal cysts can be firm but are not part of the testicle. This can be difficult to distinguish if you aren't a trained specialist.