This is the "Switch edge Test". You can toggle through each switch on the playfield, and it will tell you what was the last switch. Test the switch you think is faulty.
Thanks Nino, I have seen the "Switch Edges" test but to be honest I do not understand what it is used for. How do I toggle through each switch, and what do you mean by "it will tell you what was the last switch"? Is an opto the same as a switch? 
Matthew
As mentioned by the others - just press any switch on the playfield and it will register on the display. Cross reference the switch name with the actual switch you are depressing. Find the opto in question and interrupt the opto - it SHOULD register on the display. The last switch will remain on the display until you press the next playfield switch.
