Testicular self-examination helps catch testicular cancer early, which matters because it most often affects men aged roughly 15 to 45 and is highly treatable when found early. Warmth relaxes the scrotum and makes the exam easier, so do it after a warm shower.
Hold one testicle at a time and gently roll it between your thumb and fingers, feeling the whole surface. It is normal to feel the epididymis, a soft tube-like structure at the back. You are looking for a hard, usually painless lump on the testicle itself, swelling, a change in size or firmness, or a heavy or aching feeling. The two testicles are often slightly different in size, which is normal; what matters is a change.
See a doctor promptly if you find a lump, swelling, or new change, even if it is painless, since painless lumps are exactly the ones to take seriously. Seek urgent care for sudden severe testicular pain, which can signal testicular torsion, an emergency requiring surgery within hours.