You can actually wack one pretty good with the handle of a plastic screwdriver, just donít use a sharp edged metal tool.
If it operates, youíll hear an unmistakable Whoosh as the gas changes paths.
Before you do anything though, find it and have someone actuate the thermostat full range and wait for say 1 min at each full range, at one of those you should hear a click, thatís telling you the solenoid is getting power and the solenoid is stuck if you donít hear that Whoosh.
If it doesnít click at all, then you may have an electrical
problem, if it clicks but no whoosh, the reversing valve may be stuck.
I believe itís usual for the no power condition to be cooling
and it takes power to cause one to go to heat.
Stuck reversing valves are pretty common as they can sit for long periods and eventually get stuck, very often a percussion adjustment gets them working again.
Personally for some reason, Iíve not heard of one getting stuck in the heat mode, not sure why though, but they always seem to stick in the cooling
mode. Maybe thatís because when not operating they return to the cooling position as thatís where the solenoid goes when power is off, at least for the ones Iíve had.