How can you tell if a mountain valley was formed by a stream or glacier.

Water flowing in a stream erodes through abrasion, saltation, traction of material along the river bed etc, cutting down into the ground mostly just below it, creating a V shaped valley.

Ice in glaciers scours its way through the rock material beanth it. Because of this bulldozer kind of effect, it creates a U shaped valley.

Well all you will have todo is look at the shape of the mountain and the color