I don't get why people say they don't breed in freshwater. I put mine in a freshwater tank and they have laid eggs so they can certainly mate. It's just that their eggs do not hatch in freshwater. You have to move them to sea water for them to hatch, but from a post I read, even that might not be correct... it might just require a water really high in KH and GH to hatch regardless of the presence of salt.
Anyway, to answer your question... you'll find out if you see eggs on your tank. Mine laid them on a piece of stone I have.