Can my doctor tell if i have a hydrocele by doing an ultrasound?

Yes: Yes, a scrotal ultrasound will confirm the presence of a hydrocele. It will also be able to identify any other abnormalities that may be present in the scrotum as well as evaluate the testicles and epididymides.

Or a flashlight: A handy way to tell a hydrocele from a tumor is to shine a flashlight behind it. If it's a hydrocele, the light will shine through the mass whereas it will not shine through a tumor. A handy, quick-and-dirty way to tell if a scrotal mass is a hydrocele.