I've read that the length of the fin may help, but they also tend to get nipped, so it's not that reliable... I've also read that the male carries his more angled than the females that keep it at a right angle, but I haven't noticed this.
I actually started with three, so I could hopefully have a pair. It may take a couple months depending on their age etc, but just let them be and wait for them to pair off on their own. After a while you will notice one of the three is not as welcomed, starts hiding in corners and stuff while the other two pal around. This has the added benefit of the better of the two being naturally chosen as the mate. Then you can return the third wheel or get more to try and pair it up also!