It's pretty much impossible to sex even adult oscars without checking under the hood. I've heard that you can "vent" them, which involves taking them out of the water, flipping them upside-down, and looking in the vent to see if the fish has male or female bits, but even then both sexes can look the same to the untrained eye, and it's not something I'd want to try doing, that's for sure...
So yeah, like Somethings Fishy Here said, it's basically the luck of the draw.