You can use the
getpeerinfo RPC command to get the IP address of each of the nodes you are connected to.
Once you have the above information, you can use a tool to monitor traffic between you and that IP address. There are a number of tools that can help you do this. a google search found one
tool called wireshark, and another tool called
utop. I have never used either of these tools, and have done no research on them beyond a basic google search looking for them, so use at your own risk. Note that this is measuring the actual amount of bandwidth being used, not necessarily the capacity to transfer data between your node and the nodes you are connected to.