How can I monitor the usage of NVLink connections?
nvidia-smi nvlink

This command shows various information about nvlink including usage. If nvlink connections are utilized, usage should go up during training. 

nvidia-smi nvlink -h


Shows available performance counters on present cards. 

"NVLink Usage Counters" section in this tutorial shows how to see if data is being transferred across nvlink.

https://blog.exxactcorp.com/exploring-nvidia-nvlink-nvidia-smi-commands/