As far as I know, there is no easy way to get at the back of the board while it is still in the case. You can swap the opto and check the PIC while it is in place easily enough. Another thing that you can do is slightly flex the board with a non conductive item such as a dowel or wooden spoon and see if the LED flickers which is a pretty solid indicator of a cold joint but that method doesn't always find it. If the opto swap doesn't fix the channel, you would need to remove it from its case anyway to repair it.