Males usually have black markings on their arms and will be somewhat smaller. However, do not depend on size alone to determine. Males are usually the vocal ones as well.
The above comment on black markings on the arms does not by any means go for all frog species. Vocalization is a more general clue. If you should happen to find to frogs clamped together the one on top is the male.
The black markings referred to are called nuptial pads, they are used to help the male grasp the female and do not occur in all species. The majority of frogs only develop them during the breeding season anyway. Most males are smaller than females but not always. My Gargantua was a big boy and he was larger than all of the females I have seen. Any frog that calls for a mate is a male. Distress calls don't count, females can make those as well. Males are often more brightly coloured but this is not universal. And yes, if you find two frogs mating, the male is always on top.
Female frogs are generally larger, and males frogs generally have a dark patch on its feet or toes to 'grip' the female with during mating, but it varies between each species. in most species only males sing, and males and females throats may be a different colour. before you can tell the difference you will have to work out wich species your frog is.