You can get a pretty good idea quite easily, just by looking at your own family and your health. For example, if there are no childless couples on either side of your family, no miscarriages, no genetic disorders, and lots and lots of healthy children, then the chance is high that you’re also fertile.
Even more so if your health has always been good, and you exercise regularly and eat well.
And you have wet dreams, get erections easily, and ejaculate when you masturbate.
For most men, just knowing all that is enough to stop worrying.
But your fertility will probably be lower if you’re obese, if you smoke, or drink too much alcohol. Or wear really tight-fitting underwear. Or use anabolic steroids, marijuana, cocaine, meth, or opioids such as heroin, morphine or fentanyl.
There’s a chance you might be permanently infertile if you’ve ever had mumps. Or a serious urinary or sexually transmitted infection. Or surgery around the genital area, such as a hernia repair. Or an unsuccessfully reversed vasectomy. Or torsions of the testes that weren’t treated soon enough. Or chemotherapy or radiotherapy.
If anything like these applies to you, then you could have your sperm checked. A laboratory can measure your semen volume, total sperm count, and concentration. They can also assess your sperm’s vitality, shape and size, and their ability to swim efficiently. If those are all normal, then there’s a high probability that you’re fertile.
You could also be tested to rule out genetic disorders such as Klinefelter's syndrome (an extra X chromosome), Kallmann syndrome (failure to start or complete puberty), testicular feminisation, testicular cancer, injuries to the testes, genital abnormalities, and pituitary tumours, for example.
But no matter how hard you try, you simply can’t be completely certain without a ‘real life test.’
Because no lab can rule out all the possible causes of infertility – some of them haven’t even been discovered yet!
Okay, I suppose that in principle a lab could fertilise an egg with your sperm in a petri dish (‘in-vitro fertilisation’, commonly abbreviated as IVF). But even if that was successful, you still couldn’t be sure that the embryo would grow successfully. Because there might be a fault in one of your genes that would prevent its development.
So ultimately there’s only one way you can be completely sure that you’re fertile: to actually have a child.