Just make sure that they are active, their fins are not clamped up, no white spots, and they are not behaving abnormally (things like aggression).
You can tell it's a female if the small fin on it's belly area is the shape of a sea-shell, if it's in the shape of a sword, it's a male. And males are generally more colourful. ;)
10 Gallon Tank:
~Countless Pond Snails~1 Espe's Rasbora~1 Harlequin Rasbora~
5 and 5.5 Gallon Tanks:
Avatar by Fenghuang
Many of my scaly friends from my original tank from 2011 have passed, except for two! I am in the process of re-doing my tank. :)