How does a guy know if he is fertile or not?

For the purposes of safe sex and to avoid pregnancy always, always assume that you are fertile. To seriously answer this question....Unless you have had very serious childhood disease or if you have a genetic abnormality then assume you are fertile. I have seen men in their 40's who have been sexually active since their teens and they've never gotten anyone pregnant that they know of. And then "boom" their wives or girlfriends are pregnant. Unless you have had a sperm count and you know for an absolute fact that you are infertile....Then you are fertile. Most men are healthy (including reproductive health), until proven otherwise. As stated above, if you have had no serious illness, such as mumps, or injury to the testicles causing serious swelling then you are probably normal. If you have had unprotected sex on a regular basis for a year with a woman and no pregnancy has resulted then one of you may have a fertility problem. If she is known to be fertile and has had no change in her reproductive health then a semen analysis may be indicated to see if you are normal. Answer: I think you can simply ask your family physician for a fertility test or to have your semen counted.