Just checking that the red light is visible at the other end shows that some light is passing through the cable (it may be complete transmission or just a small amount of light). Its presence means that the cable 'may be' working. If there is no light passing through, then it is dead for sure.
Check it with the audio signal and if everything works then it is fine. If not, it is dead.
It's a solid fibre, most often made of a kind of plastic. Yes, it is pretty sturdy, but too much bend can and does affect it's light carrying ability. I thought I had broken one once, but it was just the box had got pushed back and there was too tight a bend.