By gunked up he means internally with the little vanes that push the fuel around and the piston in the back that builds the pressure. Oh, and all the little pieces that tend to get a coating on the metal from years of non use. Only way to tell if they kind of work is fill them up, hook up an input and output fuel line and run a drill on the front nut.
These things turn at half the revolutions of the engine so getting one to spin at 500 rpm is good enough most times. You are looking for little short spurts of diesel coming out the check valves in the back of the pump. cover with a clear jar or plastic to keep it from flying all over the place, because that is what it will do.