If you are male, you could have your semen analysed. It is a simple test that will tell you about how many sperms there are in a sample of your semen, as well as other characteristics of the semen. It will also be able to tell you if the sperms appear normal and move normally.
If you are female, it is far more difficult to determine if you are fertile and far too complex to describe here.
At any rate, these tests can only tell you about some of the factors that affect fertility. Whether a couple can produce a child is dependent upon many factors, some of which cannot be tested. The final test of fertility remains pregnancy and delivery of a baby.