Remember the old saying "righty-tighty, lefty-loosey". Looking down on the knobs, turning clockwise should close the valve. If you want to be sure, assuming the stove top works off those tanks, turn on a stove burner and light it, then close the valve. If the burner doesn't go out try turning the valve completely the other way (counter-clockwise). Of course if the stove works off a different tank nothing will happen either way.