Loosen the rubber "T" on the side of the line going to the pump. Next, remove the bolts from the suction tube flange on the top of the pump and rotate it out and away from the pump. You'll now be able to look right into the suction port. At this point, if you start the engine, the HST will draw air from the disconnected flange, but if you don't run the engine long, it won't hurt the transmission. Start the engine and then use your hand over the pump inlet to see if it has suction. If you don't want to check with your hand, then put some oil on a flat piece of metal or something strong and see if it suctions down to the pump's inlet.
All tests in the Repair Manual are for pump pressure. I don't know of a single test for the suction side, so you'll be winging it. Without any numbers, there's no way to know for sure, but if you have significant suction at the pump, then it should draw from the reservoir. If you have a quart of fluid handy, maybe pour it into the pump as it is running and see how fast/if it pumps through. Remember, don't run too long because your HST charge pump is sucking air unless you block the flange opening of the pipe.
EDIT: You'll probably need something to block the air flow from the fan if you try to pour into the pump. Trying to pour from a bottle into the pump with the fan blowing the oil everywhere could cause a mess. A piece of cardboard should be sufficient.