Very simple - males are brightly colored, females are not. To be a little more specific, the wild male color is a bright banded blue/red combination with some speckling on the fins, females are silver-grey, occasionaly with faint yellow-brown barring. You'll also see 'powder blue' males and males with more red than blue or more blue than red or even 'flame' colored males who are predominantly orange-red. In all cases the female is duller. If that's not enough, females also appear more rounded as they are deeper-bodied. If you look down at them from above, females are also wider. Males are slimmer and more 'stream-lined' in appearance. Lastly, the males dorsal fin is a lot more pointed and longer than that of the female and the anal fin, also, in males takes on a 'bulbous' shape and is larger. If you google for some dwarf gourami pics you should soon begin to notice an obvious difference between the sexes.