Try this,

With a cold engine, take the rad cap off and adjust the coolant level to just cover the radiator core holes by appx. 1/2". ( you dont want it full because when your engine heats the level can come up and spill)

With a thermometer in the filler neck and in contact with the coolant, start engine and pay attention to temp. When thermostat is closed , you will have very little/no movement in the coolant as viewed through the neck. As the engine reaches a temp where the thermostat opens,(note this temp on your thermometer), you will see rapid coolant movement from one side of the rad to the other until it cools enough to close the thermostat.

If the thermostat is stuck open you will have a lot of fluid movement all the time.

Don't burn you self , use proper saftey measures to make sure you don't get splashed in the face with hot coolant.