"Turn the key on (do not crank), and place your hand on each pump."
The catch here is that the pump(s) will only run for a couple of seconds. This means someone else would need to turn the key.
Or, you could jump pin sockets 7 and 8 at the relay. This would make them run continuously.
Or, you could run a pair of long jumper wires from the battery to each pump and test them individually.
1986 300E 5-Speed 240k mi.