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?

Transillumination: Bright light shining through a scrotal swelling usually means that a hydrocele is present, however transillumination also occurs if there is a testicular tumor in addition to the hydrocele. Thus not a completely reliable sign and does not replace accuracy of scrotal ultrasound.

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.

No: Fluid (such as hydrocele fluid) transmits light whereas solid structures like a testis or a growth do not. You have a hydrocele if: 1. You have a scrotal mass 2. The mass transmits the light.