Testing thermostat is easy.
Remove the thermostat and place in a pot of water along with a thermometer (candy thermometer works well). Slowly increase the temperature while monitoring the thermostat valve. Note when it begins to open and fully open. It's rare these days that the thermostat fails closed. They usually are designed to fail open requiring longer warm up times.
Before doing the above, perhaps do this.
With the radiator cap removed (follow all safety precautions when removing), run the engine until the fan kicks on. Pay attention to see if and when coolant is flowing in the radiator.
Good luck and post back what you find.
2012 JD855D OPS Black Poly Roof, Deluxe Cargo Box, Front and Rear Protection, Power Lift, Front Hood Rack, Rear Screen (homemade), 800 Watt Inverter, 2500lb Winch installed in cargo bed, Front and Rear LED auxiliary lighting (4" square 27 watt), Front and Rear CV Guards, Koplin Side Mirrors