The green wire is probably the thermostat wire but you need to verify that first. So, first set your meter on the lowest ohms setting and measure from the green wire to ground to make sure it's not a ground wire. Your meter should show 1 or open. If your meter shows 1 or open, turn the thermostat on the middle (or highest) setting. Connect your meter to the white wire and the green wire. Your meter should show 0 or closed. Turn the thermostat to the off position and your meter should show 1 or open.