Doctor can evaluate
Some people have one normal and one less developed testicle.
A primary care doctor can check and examine the testicles, and of course the prostate if a guy is old enough for that. Testicles are easily scanned by ultrasound to see what's inside, to rule out tumors, etc... A urologist can follow-up if the ultrasound says anything other than normal. The ultrasound can measure the testes sizes.