...what's a good indication/symptom that my thermostat is not working properly?

My temp gauage is ok, but sitting still my heater does not blow warm air until I rev it up, then it does.

Sticky thermo?

Heater core?

 

I'd try burping the coolant system first, make sure there is no air in there at all - especially if you've just done something where you drained/removed the coolant.

 

Thanks for the reply. They found out it was the radiator cap that is failing. Got one on the way.