It is difficult to define a sex of small fishes as in certain cases females look as well as males. And if not to know features which are peculiar to this breed then at all it is impossible. But to try to distinguish them after all it is possible, especially if it is very important for you.
Address the seller if you are only going to buy small fishes. Usually in such shops people who more or less understand small fishes trade. If cannot explain you who a male and who a female, then address to other zoological shop better.
Compare a form and the size of fishes. Often females are slightly larger, than an opposite sex as bear future posterity. But there are also exceptions. If your small fishes have though any visual differences, so whitebaits shortly will appear. Periodically watch small fishes. If some suddenly began to increase in sizes, and the paunch was rounded, most likely, that it is females. You need only to wait a little to define a sex of aquarian fishes.
Pay attention to coloring of aquarium fishes. Usually color scale of males brighter and saturated - on it you will be able to define a sex of aquarian fishes. Samochki in most cases have uncommon appearance. But all this depends only on a species of fishes therefore you can be mistaken. Fins and a tail at males are slightly longer and brighter. Their color can change during the "marriage" period: there are various impregnations and additional pigmentation. So they become more attractive to females. After spawning there are return changes. Get special literature on this breed of fishes. In the book you will be able to read in detail about characteristic sexual differences. If you do not know what version your aquarian pets treat, look at their name in shop. Usually on aquariums it is specified. If you cannot define a sex of fishes with an accuracy, but at the same time you noticed a number of differences, then just wait for addition. Some species of fish visually do not differ in gender therefore be not upset if in an aquarium absolutely identical individuals float. It is possible that they are heterosexual.