Testing the optos and the opto board is fairly simple and can be achieved with a multimeter. In this particular case , if changing the opto board fixed the problem, then the actually optos were fine. There isn't a lot on this board. Diodes and resistors are super simple to test, and the only integrated circuit found on the boards are LM339 which are the most common OP amps, available at any electronic parts stores. These are also very simple to test in the way they are used. Unless there is visible damage on the board, the very first thing I would look for is a cold solder joint at any of the connectors first, then the rest of the solder joints. If all looks clean, I'd start blaming the LM339 and test it. The issues here seemed intermittent since the scenario changed during game play. This leads more towards a loose connection, if not, it's the ICs. Diodes or resistors don't cause intermittent issues, they work or don't.. unless their connection is loose.

If you wish to repair your board and need help, I can assist you.