In general yes. There are a few situations where an opto is initially blocked, in which case it would read open and then closed when you "remove" whatever is blocking the beam.

Watch the video and then do the same thing with your game: when switch A and B and C are closed then switch D falsely indicates closed. You only need three of the switches to determine the rectangle. So if you have A and B and D, you can figure out which switch is C. Finding the pattern is the hardest part; the resolution once you've done that is typically pretty easy.

As robertmee suggested pull all of the pinballs and see if the problem goes away. Then add one ball back at a time until the problem shows up again.

Be aware that things are slightly different when dealing with optos since there is no direct physical link like there is with a switch. The best way to deal with this is to remove the connectors on the opto boards (the row/column ones) and see if the problem goes away.

The article that goes with the video will provide more background info on how the switch matrix works.

http://pinballrehab.com/1-articles/solid-state-repair/repair-guides/146-switch-matrix-theory-and-troubleshooting