As with many Central American cichlids, females will show a dark area in the dorsal, and where males don't.
This dark area usually doesn't become prominent until the female becomes sexually mature though.
This is why the advice SMSChris gave is sound.
The larger the group you get, the best chance of getting a "compatible" pair.
Compatible, is the key word, just because you get a male and female, doesn't mean they will be a solid pair. Often getting a group and allowing them to select mates themselves provides for a pair with a strong bond, that doesn't deteriorate into death at the first blip.
Thorichthys are a fairly social genus of cichlids, and with the proper size tank, a shoal will be a more natural way of life for them.
Although not FMs, here are some examples of female Central and South American dorsal areas.