Is a diagnostic test to confirm the presence of a hydrocele by shining a bright light on the scrotal swelling to see the shape of the testis through the surrounding translucent excess fluid.

The answer is Transillumination

When the testicles descend into the scrotum in infants, the fluid in the scrotum is usually reabsorbed into the body within a year, and the channel for this fluid closes. However, when this reabsorption and closure of the opening fails, a hydrocele forms.
To diagnose a hydrocele, a physical examination by a physician is done by shining light through the scrotum to check for the presence of fluid.