Both men and their female partners should familiarize themselves with the normal anatomical features of the testicle. Many testicular tumors (and breast cancer tumors BTW) are found by one's sexual partner.

If a lump is found or if one testicle is significantly harder than the other, you should visit a doctor immediately. The quicker you know, the better it will be for you in the long run.

My wife noticed the change in my testicle first. I noticed later. My wife estimates she noticed the change about 6 months before I was diagnosed, but we didn't know anything about testicular cancer. I was 19, and hardly on the lookout for cancer. I eventually went to the emergency room suffering SEVERE pain in my abdomen: this was later determined to be caused by metastasis displacing nerves and blood vessels near my spinal column. Don't wait that long: you may live through it, you don't want to ever experience pain like I did.