Verify that the solenoid is getting juice from the computer. If its not, then I would suspect the sensors you talked about.
But... 9 times out of 10 its the relay. Even if you buy one and find out that its not the problem, it will be someday and you'll have a spare
Glow plug relays are like Kryptonite to the powerstroke. The engine will run forever... as long as you have a spare GPR.