How will you be able to tell if a testicle is shrinking?

Measure it: The vast majority of men have somewhat unequal sized testicles and they do shrink somewhat as we get older. Certain disease can cause them to shrink such as pituitary failure. Testosterone therapy can also shrink them, as with anabolic steroid abuse/use over prolonged period of time. If they get smaller over short period of time, consult doc. Good luck.