I was recently having a conversation with a colleague about doing some troubleshooting on a fiber optic network connection he was working on, and was looking for a way to verify light was actually making it down the fiber.
There are of course specialized tools that will measure this, and you can also get inexpensive ‘business cards’ with a coating that lights up visibly when you shine IR on them. However, even better is something you have with you all the time. Your smartphone!
The camera in almost all smartphones is at least somewhat sensitive to IR light, so you can do a quick check using your phone. This trick also works for TV remotes if you’re not sure if the batteries are dead.
On an LC connector like this, you should expect to see light on one side, and the dark one is for the return signal from the optic you’re plugging it into.
Just be careful not to look at the laser with your remaining good eye!