I am not going to get into removing the door panel on this DIY. There are multiple write-ups on removing the door panel.
Anyways, once you get the door panel apart and you want to make sure that its a voltage problem or a switch, this DIY is how to test the switch to make sure its working properly.
Its very very simple to test each switch. I will show you how to test the passengers side, passengers rear, drivers rear switches and I will post a diagram of how to test the Drivers side master switch.
Real simple, hook your multimeter into the pins as shown in the diagram above. Set your multimeter to audiable continuity and push the switch in the direction your testing.
To test the master switch, its the same exact concept, but you test a lot more pins: