So, if your sensor/sending unit is giving wrong data, your check engine light should be lit. If for some reason that didn't happen another way to check is by disconnecting the temp sensor altogether.
Based on the symptoms you've listed there can only be one of two:
- Bad Temp Sensor
- Bad cluster (probably with a random fried resistor somewhere on it)
Disconnecting the sensor or throwing a new one in is the best (and easiest) way to verify if the problem lies in the sensor. If that doesn't resolve it, then process of elimination would dictate that the gauge is the problem.
I 100% agree that avoiding tearing into the cluster is ideal, so check the sensor as a first step. This won't require any special tools or anything.
Starting the truck with no sensor connected could cause some odd, annoying though non-damaging behavior. So I'd recommend throwing a new one on rather than just disconnecting. That said, if you're only interested in the gauge action, then you'll only need to run the truck for like 30 seconds or so to see what it does.
Only reason for mentioning this, is don't run it for an extended period of time. While running the motor like that won't be particularly dangerous, driving it as usual without a temp sensor attached would be a very risky endeavor.
Why this conclusion?
Simply, any overheat condition wherein the motor is actually over heated, there will be other symptoms. Especially if it were actually overheating that much.