I'm pretty sure I don't have a heater valve but I guess the easy test would be to set the heat control to cold then feel the heater core hoses on the firewall, if there is a heater valve, and it is operating properly, then one hose will be much cooler than the other. If no valve then both hoses should be the close to the same temp.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles