Coolant heater flow direction help

If you have another pair of hands -

Disconnect your bunk lines that are looped together, put in a small bucket or pan.

Plug both ends without clamping, bolt/plug etc.

Have your helper bump the starter, or run for a second - the one that pukes out with pressure will be your supply.

If the engine is shut off while the diesel fired heater is in operation, then would it matter which is supply/return?

I use a block engine heater in a main coolant manifold powered by the generator, it's simply heating coolant in the system regardless of any flow, as the engine coolant pump is not in operation.