Or are you referring to electric blue rams, Mikrogeophagus ramirezi? Whenever possible, use/include scientific names for proper identification. If it is, males tend to have an elongated dorsal fin. That said, not a great system...
Just saw your former post. It is a Electric blue ram. Fluttermoth did give an answer though.
I know those are the normal colouration, rather than electric blues, but if you look closely (and it is a subtle difference; rams are very difficult to sex), you can see the difference in finnage on them. The anal fin reaching out past the beginning of the tail is the easiest thing to look for.
Do bear in mind that immature or sub-dominant males may look female for a long time.