I assume your bilge pumps are not far from the stern, which is typical placement. When the boat is in the slip at the marina we just use the dock hose to pump in a few gallons of water into the boat to test the auto pump float on the small automatic 750 gph’(less likely to kill the battery) rainwater pump. The big main pump is manual. It should not take much water at all but if the water is moving forward, just have two or three big guys stand in the stern and it should take less water to activate/test the pump. You want to hear and see the water pumping out the side of the boat for each pumps outlet .
You still need to physically access the pumps so that you can clean the strainers and impeller compartment and get out all the crap that ends up in them including little bits of mono line trim that can get wrapped around the impeller. Cleaning the pumps and testing them with actual water in the bilge is an annual beginning of season ritual for us. My Rule pumps have release clips that allow you to pull the pump off the hull mounted part to inspect and clean them. Your pumps may be working but may not pump as much water if they are clogged up with bilge crap. It also helps to clean the bilge once in a while as there will be less crap to potentially get sucked in and clog the pumps in an emergency.