Well, basically, what you need to know is that the diverter will only work for the third ball .The first two will be considered locked just by passing on the left ramp.
During the game, when the ball goes on the left ramp, there should be a motorbike sound, with a little video with it. If you don't have it, your opto isn't functional indeed.
One good technique to verify it is to use a digital camera : shooting around the playfield, you should see white lights coming out of the optos. On mine, the one that was a problem didn't show no light.
In the end, i just unscrewed and took away the left ramp (5 minutes) and realized one wire was not connected anymore.
But you may have to change the led, if that's what is dead.
If you don't have one and you don't want to spend your money on shipping fees, you can do it with an old remote control. Just dismantle it, take the opto and put it instead if the one that's not working.