How old is the cockeral in question?
There is no 'age' that indicates a rooster is fertile. Not all roosters are fertile, you know, same as with people. You just have to hope for the best until they prove themselves.
Put him in with your hens. In about a week or 2 crack a few eggs and look for the bullseye ring on the yolk indicating eggs are being fertilzed. A small white spot (germinal disc) on the yolk can be seen even in infertile eggs. The tale tell sign is one with another ring around it, indictaing fertility and very early stages of a possible embryo.
Then you can further test by incubating a few and seeing how they develop.
edited to add - a sexually active rooster does not mean he is fertile and able to fertilize the eggs. Some hens with very much fluff around their vent need to be trimmed up so contact can be made.