It's supposed to close when cold, and open when hot. My Ford manual says 112 degrees F in one place, and 144 degrees F in another...
You can test it by seeing if it is passing 12v through to the cold advance solenoid when cold, and not when hot. (assuming that it is getting 12v...)

 

We hooked up a ohm meter to the sensor switch. Then used a heat gun to warm it up. Once it hit 100degF on my infrared gun, the ohm meter read infinity.

 

What is the nature of the problem you are having with the high idle solenoid?

I don't know how accurate your infrared gun is at that temperature range or if the switch was removed from the truck when you tested it or while it was in place (would make a difference) but if the engine coolant was in fact 'cold' or near the ambient outdoor temperature then your test seems to show that the sensor is working, possibly opening too soon if that can be a problem with these. I'm only guessing that it has a bi-metal strip with a contact inside and if the contact has been worn or corroded it could open the circuit too soon.

Some people don't know the following, some of which I didn't know until recently: Senders and sensors that are grounded through the threads need to have good continuity to ground so I would not recommend using typical anti-seize compounds on those unless labelled as electrically conductive or "Sensor Safe" and definitely don't use teflon based anti-seize paste or teflon tape.

Teflon is a non-conductive electrical insulator (except for a very few special formulations). [NEVER use teflon tape on an acetylene fitting - the chemical reaction over time can be dangerous]. The old silver color anti-seize is what I have always used, i think it has aluminum flakes in it for some conductivity but that's still not good enough. I also have the white teflon paste which I've used before and shouldn't have. I've been making those mistakes for a long long time without being aware of the conductivity issues until recently. I bought a plastic container of COPPER based anti-seize a few weeks at the local O'Riellys Auto Parts store, 'VersaChem' 'Type 13 Sensor Safe' ...'resists temperatures up to +2000 deg.F. I already know that generically, the copper in it provides better conductivity that the silver colored pastes with aluminum flakes.

Back to the Idle Solenoid: The 87 manual states that in order to extend the rod to high idle position you need to depress the accelerator pedal once during the glow plug cycle before starting. Otherwise it won't extend until you rev up the engine or depress the pedal to start driving.

Mine was staying on high idle position all the time, even after driving it for 60 miles on the highway and I had to manually push the rod back to low idle position after I thought the engine was warmed up enough. I thought it might be corroded and sticking so I cleaned it out so it would move more freely. I think I used carburetor cleaner or brake cleaner followed by some WD-40 and I should have thought to use a spray silicone lubricant instead of solvent based WD40 - reason is my concern about solvents affecting the very thin clear electrical insulating coating on the wires of coil windings whether in a solenoid, motor or generator.

I was also trying to get my temperature gauge to work and replaced both the temperature sender and high temp. switch on the driver's side - gauge still barely registered any movement (moved up 1/8 inch or so from the bottom after the engine was warmed up)

The final clue was that I was getting very little heat in the cab while driving down the highway one night. The cause of all of the problems mentioned was that the thermostat was stuck open and the engine never warmed up enough for the gauge to move more than 1/8 inch or the idle solenoid to return to low idle. After replacing the thermostat (Motorcraft) all of those things worked right.

The point of that is, the actual engine coolant temperature and the rate that it warms up will determine when the temperature switch will open the circuit and the coolant temperature/rate of warm-up is dependent upon the thermostat working properly.

Back to my question: I'm curious: What is the problem are you having with the fast idle solenoid?