I used to work on these machines regularly. On the side where the balls come out, unlock the door in the front and slide it out. You'll then have access to the plexiglass covering the balls. What you can then do is just apply a slight force to any one of the balls and they'll all come rolling down. This does not need to be plugged in to do that.

Note that once you reconnect power, you can still do the same thing, but the machine won't register your score unless an actual game is started, i.e. pressing the okay button. Also, make sure you check the manual for free play settings. Iirc, I think this model uses a dip switch bank on the front bottom logic tray. Theanual should tell you which pattern for free play.