Apple - How can I see bandwidth usage on a Mac?

Built-in Activity Monitor (Applications → Utilities) shows you network usage. Also you can see open network ports for each running process.

You can try nettop(1) in the Terminal. It is pre-installed, and refreshes every few seconds to provide a dashboard of all open network connections with their usage. Better than lsof since it shows the usage data too.

$ nettop

Then hit 'd' and look for odd-looking entries or entries with consistently large traffic in the 'bytes in' or 'bytes out' column. The 'd' instructs nettop to only show differences in each screen refresh.

Practical usage notes:

If you don't recognize the process name, Google it.

If you don't want the process around, get the pid (the number next to the process name in nettop), and kill it with kill -9 <pid>. If that doesn't solve it, find out if you can uninstall the process.

If you're interested in what the process is transmitting over the network, use the client port number for that particular connection (e.g. '53133' from 10.0.0.1:53133->74.125.68.100:80) to run a tcpdump(1) to see the data in the packets being exchanged: sudo tcpdump -nnvvXSs 1514 port 53133

If you're interested in the destination the process is talking to, but the destination IP address doesn't have a reverse DNS hostname (like in the previous example), then try visiting that IP address in a web browser as https://IP, click the broken lock icon in the address bar, and view the certificate details to find out which domain is served there (*.google.com in this case). This won't work if SSL port 443 isn't open on the destination.

Perhaps Little Snitch might fit your needs. It's a paid app though, so you can try it out using it's trial version.