Ah, so it's basically a crapshoot. I tried to gather info on sexing them and at most I got;
1) males MIGHT have a longer "banner" fin. But in a community tank, most likely all the banners have been "trimmed" by other fish
2) females have a more rounded jawline, where males have a more angular jaw.
So since I have no frame of reference except pics on the internet, I figured that mine MIGHT be male. So if I go to the store, maybe I can use those clues to pick a higher-probabiliy female. Then when I get home I'll have either a f/m pair or a f/f pair. But if I pick a suspected male for home, then I'll end up with m/f or m/m. And from what I can tell, getting m/m is worst case scenario due to some territorialism. So if I could pick a best-guess female, I'd have a better chance. Sort of.
But I guess that I can just keep adding them until I find a pair, as every time I buy one is a 50% chance of pairing up the one(s) in the tank. Interestingly, (when buying a single fish) the chance of a pair is always 50% regardless of how many are already in the tank!
V