possable capacitor?

I have an electric motor on my air compressor that is not working. When you put power to it, you hear that distinct sound of power going to it but it doesn't move. Sounds like it is trying to move but can't. I can spin the wheel with one finger so its not bound up. Someone had mentioned that it might be either the start or run capacitor. Can anyone explain to me how to test the capacitor with a multimeter? I don't have a capacitor tester and I saw on youtube where they say you can test them with a multimeter but not in very good detail. I am lost here and I don't feel like getting a new capacitor if thats not my problem. Any help would be greatly appriciated.