It can be hard to tell, I have hundreds of fighters and in one of my tanks I have females and ? (they're the ones I don't know what they are).
I had what I thought was a definate male a couple of days ago going around attacking my other 4 mth old males so I jarred him (or so I thought) to walk out to the room last night in time to see him! dropping eggs everywhere. My IT fish as I call them, if people want them I very clearly tell them I don't know but in saying that the bulk I can tell whether they're male or female.