The best all round technique for determining a female is the bulge in her belly when fed well. This technique applies to almost all tetras, barbs, and Rasbora . If you aren't sure, capture some fish in a net or a bag and look straight down. Often you can see a bulge distinguishing the females which are ready to spawn. But as I will repeat in this article, check the books and articles on a species for tips.
Sexual dimorphism varies per species.
I recommend any and all of the Baensch Atlas.