There’s that number (3 in the lecture) that shows whether the material was assigned to a material slot. That’s also what that “0” you see in the Material Browser means. However this doesn’t say anything about whether this material slot was used on any mesh-object’s faces.
You could have two materials, Red and White, have them both assigned to material slots, but have only Red assigned to the actual mesh. The whole cube is coloured red. The White material wasn’t used.
So I was wondering if it’s possible to see whether a material slot is used on faces of the actual model. In more complex scenes with more materials and more mesh, it would be useful to know whether all of your materials were actually used on your model.