When you are making a custom faction and you are choosing a race like Karth or Man, there is no way to see what the races do.

There should be a tooltip, or it should show the little race icon on your picture like it does when you leave the race selection, custom faction screen.

Now you have to make all the choices for your race, advantages and disadvantages and then race and then save it creating a new custom faction. Then hit accept to see what your race does, maybe it will give you golems or slaves or +10% exp, or +3 attack and init, who knows!

Mike.