I've heard that if you shine a strong light through your sac and see it on the other side then it is a hydrocele. Is this accurate?
Perhaps. In general, hydroceles will "transilluminate" and this test helps with the diagnosis. It is not foolproof and since hydroceles can also occur with other pathologies it needs to be assessed by a urologist. Scrotal ultrasound is a far more accurate means of confirming the diagnosis and ruling out other pathology.