electric window problems

[66gmcguy]

Hey all. Just picked up a 1990 Chevy R3500 crew cab dually. it's pretty much a fully loaded Silverado edition. Neither of the front windows work. The passenger window doesn't work when I try rolling it down from the switch on the passenger door. the rear windows work from the drivers switch. Before I start replacing stuff I don't need to, how should I test if the problem is in the switch, the wiring, or the regulators? Thanks all