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