You may want to hound cichfreak with some questions

he's quite experienced. Anyway, here's my tips, if you notice two oscars liplocking AND tail-slapping(each facing opposite ways slapping their tails together), and maybe tail quivering then you may have a pair forming. If instead of liplocking what you see is "continuous" brutal tearing at each other then you're likely seeing two males who don't like each other. This type of fighting can get quite violent beware! Males do have more color, usually also bit more "saturated" color, pattern may extend higher up, ie more red goes toward top of dorsal fin. Male may have dark spots right under dorsal fin surrounded by red, ie "ocelli" under dorsal fins. Sometimes about 3 of them under dorsal fin. Females may have these dark spots, but not always visible or more "faint" and usually not surrounded by much red pigment at all, just faint black spots, female head will tend to be smaller, thinner, male more roundish fatter. Male will have bigger fins, longer pectoral points, bigger dorsal, sometimes overlapping tail fin abit. End of dorsal fin on mail is sometimes more pointed than female. Yet some males will have quite rounded dorsal fins at the end but they will be noticeably bigger than female counterpart and overlap more of tail fin. There is also something about the slant "angle" of the mouth but won't go into that. Also, probably easiest is to look at some pics and try judging by genital papillae. In other words, if it looks like an "ovipositor" , it's likely a female. Female has wider, barrel shapped genital papillae because the ovipositor needs to pass through that ~2mm egg. (Usually points more 'straight' on female and more slanted on male oscar.) Others can provide give you some links to good articles on sexing by venting.