There appears to be no way to detect whether a soldier/turret/gunner/whatchamahave is in process of reloading a magazine (without resorting to muzzle EHs), so dedicated command returning boolean would be due. 🤔
And to make it more versatile it should accept weapon/muzzle as argument as well, so something like:

isReloading <unit>; //just generic if-any-held/operated-weapon-is-being-reloaded check
isReloading [<unit>, "Secondary"]; //muzzle specific check