I can tell two ways: one is that it smells like semen, lol. Not really sure how to describe it other than I know what my husband's semen smells like so I take a whiff and that will tell me. The other way is that semen is not lubricative so if you rub it between your forefinger and your thumb and it starts to dry after a bit, it's semen. EWCM won't have much, if any, odor and will not dry with friction.