Measuring network bandwidth usage
There are a tonne of options for monitoring bandwidth usage on Linux, one very quick one that springs to mind is bmon. This is a very easy program to use and install with a nice interface.
There are more however:
- nload
- iftop
- iptraf
- tcptrack (to name a few)
If you're however looking for a more long term, permanent solution then I would personally use Prometheus + Node Exporter and Grafana for visualization - a simple guide to get you going.