Found this in "The Guide to Owning Oscars" book by Richard F. Stratton.
I've never tried to look for myself but this is what the book says.
"It is possible to ascertain the sex of oscars, but it involves netting these large animals out of the tank and holding one of these stuggling behomoths in a net while utilizing a flashlight to closely examine the genital area of the fish. ........ the male's spawning tube points back toward the tail and is located just in front of the opening of the anus. The female spawning tube is less pointed and points straight down. For this reason, the female's genital area gives the impression of two openings (including the anus) and this mehod of determining the sex of certain fish species has sometimes been called the "one-hole-two-hole" method.:
Hope this helps you. Good luck!