How Do You Know If A Fish Is A Girl Or A Boy?

Depends very much on the species.

Many are dimorphic -- they have quite different body shapes or colours, depending on gender.  Male guppies are small and colourful, for instance, while females are large, grey and relatively dull looking.

Many freshwater tropical aquarium fish (like guppies, goldfish, Betas) are obviously dimorphic.

Most other fish, especially adapted to live in cold water, are not dimorphic; you can't tell by looking at them.  You could tell if you dissected them and knew what to look for (how to tell ovaries from male organs), but, that is specialist knowledge. And not much help if you like your live fish.  Although it might be fun next time you get a whole specimen from the fishmonger.

Otherwise, you may get clues from watching their behaviour.  Again, this varies by species of fish.

Tips for sexing goldfish.

Tips for sexing other tropical freshwater aquarium fish.
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Well, its not easy to tell but in order to tell i need to know what type of fish it is

They have goop