open the coin door, look on the right side. should be a small panel with a slide switch and red service credit button. flip the slide switch and you should be in test mode.
be mindful you need the slam tilt switch to exit the input test, if in the event you cut the wires there to prevent it from locking up when you "tilt"
Note that there are 4 buttons, and each button has its own function (machine gun, oil slick, smoke screen, and missiles)... sometimes people think the left trigger isn't working, expecting that to be machine gun, like the right trigger (it's actually missiles).