When i was in my last year of high school, all the guys in my year level had to watch a video about testicular cancer (where a naked man was actually showing you how to check, it made quite a few uncomfortable hehe) and then we were given a type of prosthetic scrotum where the testicles had 4 lumps, and we had to find them all, the first two were easy because they just stuck out, but the last two were very tricky. Goes to show that it's best to properly feel around if you're ever concerned.