With most tail types (veil, crown, delta, superdelta, halfmoon, double), it is very easy to tell. The males have much longer tails, bigger fins, longer ventrals and gill beards that extend all the way around under their chins when they flare. Typically, females will have little short tails, short ventrals and small beards that only appear to the sides, not under the chin.
However, there is a tail type called plakat, in which the males have short fins too. Then you have to go by things like length of the ventrals, shape of the anal fin and gill beard alone, not by fin length.
There are also indicators, but they aren't definite. For instance, males are more likely to bubblenest, but sometimes ladies will too. Females generally have an ovipostor, or eggspot, between the ventral fins, but sometimes a juvenile male will as well.
Post a picture and we'll be able to tell you his gender, colour and official tail type. :)