Shine the light in one of the cable, with the other end up to your ear. If you can see the light in a mirror on the other side of your head, it's working fine.
I've done simple test using my torchlight from the other end and I can see light coming out from the other end, but I am not 100% sure because these is not a standard testing method, any advise what are the standard tsting method.
Dunno. What does the Test Specification have to say on the topic?
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856