fish different

does any one know how to tell the different between male and female; mollys,goldfish,platys,tiger barbs or any barb.

Look at the bottom back single fin under the rear of the belly. In male mollys and platys it is pointed and in females it is fanned out. I don't know if this is the same for barbs.

It is different for almost every barb. Though normally the same idea, nose, fin, change color.

In others it is distinct fin size/shape differences

For some there are the color is females will be drab while males get a more vibrant color. It can be hard to tell when they are young however, like many other fish they don't develop full colors until they start to become sexual mature.

Your best bet is to pick out what kind of fish you are trying to sex and just Google search it. There are many places that have full articles about sexing or general information including sexing of nearly every fish.