Seriously, they're pretty near impossible to sex unless they're already paired, I'm not even sure venting them is all that reliable. Most cichlids are fairly easy to sex; sadly oscars (and uaru, some others) are not.
In my experience males are often a bit more actice and agressive.
They also seems to have a more compact head and generally looks more "macho" (that is if the fish you compare with is a female of course).
This way of telleing the sex is far from safe and it also demands a bit experience with the species.
Venting them is a rather safe way of telling but it is not easy if the fish is large.
I never bother to do this is the pair is rather large and go well together.
No fighting just a bit of "showing off" mostly means that there is a male and a female.
Also a pair will hang on to each other "all the time".