Engine warms very slowly in warm weather and rarely gets to the 12:00 position on the gauge. Heat output in the cab is fair but not hot when needed during cooler weather. I suspect the thermostat is bad but it is expensive. anyone have any tips on testing the thermostat before I order a new one?

Sounds as if it is stuck "OPEN". You'll have to remove it and look inside - if it is stuck open, you'll see it.

Another possibility is that you're radiator cap is bad and the system is not building the pressure needed to funtion correctly.