Remove from vehicle.
Note position of valve in when cool in your hand.
Immerse in a saucepan full of water, on stove, and boil. If operating correctly, you will see valve in opposite position from original (cool) observation.
Remove from boiling water, carefully, with tongs or long nose pliers and watch valve position slowly return to cool position.
If radiator gets warm very fast. And you can't get normal temperature in your care and Your motor work temperature gets <90 degrees while you are driving.