Found myself troubleshooting a pesky fibre connection that wouldn’t come up. I was looking for a command that would show me if a light was being received on the interface and found these beauties:
show interfaces diagnostics optics xe-4/1/0 show chassis pic fpc-slot 4 pic-slot 1
The first shows information on light levels on the relevant optic. The second will help you figure out what type of cabling you need to be using. Handy when you don’t know if it should be single or multi mode.