As jflowers says, the tail fin will be shaped like the Ace of Spades for a male, whilst the female's isn't pointed. Females are also broader looking at them from above, but of course this only helps if you've two to compare. Females are (at least mine are) brighter and have a brighter pink belly. You may have to wait till the tail grows back though before you can tell for sure. My male is also more streamlined in shape and the females are rounder.
At a guess I would say the fish in the pic is male, but no guarantees!
HTH
Sarah