Tempture gauge reading normal, not low? Possible stuck open T-stat. Any time there is a heating or cooling problem, replace the thermostat first before any other actual work begins. Do the testing of course but if unsure about the thermostat, replace it.
Heater cut off valve under hood ok? Vacuum hose hooked up to it and the valve is opening so hot water will go into the heater core? This valve is right at the firewall so hoses may be hot up to that point. Yes the actuator for the blend door can be tested, i use a scanner! Try www.alldatadiy.com also search youtube for a video. Too much typing to explain. Good luck with the repair.
If you touch the heater hoses are they both hot (too hot to hold onto) or is one hot and one cooler (or cold) Coolant flow has to be fast enough that the hoses should be too hot for you to hold onto both of them, if they are then you likely have a problem with the heater blend door in the plenum assembly. If one hose is hot and the other is cold then you don't have good flow through the heater core because of a blockage or an air pocket in the heater core.
The temperature blend door actuator is on top of the plenum behind the glove box. You should be able to see it with the glove box removed and watch the shaft turn as you select hot and then cold on the control head. The majority of the diagnostics are best done with a capable scan tool that can display the HVAC data. The actuator has a potentiometer inside it that returns a door position signal. If the actuator is responding to commands then the door itself will need to be checked to make sure it isn't broken.