By jumpering the two wire connector to the distributor. I usually use a pair of needle nose pliers. If it throws sparks, you know the box and coil are good.
I recently sent a 6AL box back to MSD for repair. $46 and came back all good to go with all new wires and a test card.
Black 1985 GT, 408w, Just into the 10's.
Yellow 1973 Mustang Mach 1
Black 2012 5.0 GT, 6-speed, Brembo brakes, 3.73's
Wimbledon White 1966 F-100 Shortbed Styleside, 352, Tremec 3550, FiTech EFI