I have a 2001 F550. I no longer get a wait to start light on the dash. On cold days, well cold for Florida the truck doesn't start. All I get is white smoke out of the exhaust. After it warms up the truck starts and runs fine. I'm thinking either the glow plug relay or the oil temperature sensor has gone bad. Is there a way to check before I start throwing parts at it?

 

Here's the pinpoint test procedure from the tech section. It should have the info you're after.

 

The wait to start light is independent of the glow plug operation. I can see how if your Oil Temp sender failed high it would inhibit the GPR operation and give the symptoms your describing.

Do you have any way to monitor the EOT PID to see if it's accurate (Forscan, Scanguage, SCT etc...)?
If not you should be able to disconnect it and measure it's resistance to see if it's working. Even if it's working there is always the possibility that there is a wire issue, thus checking PID values is a good place to start.

I'm not sure how similar the F550 is to the light duty trucks (I know the turbo is different), but on the light duty stuff the GPR is triggered by a ground signal from the ECU.
One side of the trigger circuit is Key on power, the other is the ground leg from the ECU.
The larger lugs are the load and supply, one side should be hot always and the other hot only when triggered by the ecu with key on.
You can check operation by function testing the GPR (disconnect trigger leads and supply hot and ground, see if both load terminals go batt pos),
and/or checking for batt pos on both load terminals immediately after switching key on. A test light or other remote 12v signal device is handy if doing this alone.

There are loads of GPR failures, I don't recall hearing of an EOT failure.