Here is what I usually do. Most of the time it's either the rotary connection or the clutch itself is burnt out. I unplug the clutch at the rotary connection and using booster cables apply power to the clutch. It doesn't matter which wire you hook to positive or negative. If the clutch works with the booster cables then the problem is in the wiring or the rotary connection. By rotary connection I mean the little black round box that is on the end of the shaft that has the clutch on it. There are a set of brushes in there that let the current go to the clutch.
Also when you have the electrical connection apart put a test light on the connections coming from the switch. If the connections are all good the test light should light up when you turn on the clutch switch in the cab. If the light does not go on then check for power before the rotary connection and at the switch. You should be able to narrow down your problem with a few tests. I'm going to assume you've already checked the fuse or the circuit breaker.