Your modem will likely keep connection statistics which will tell you how much data was downloaded and uploaded.

Ookla speedtest.net will give you an indication of your network speed. I have a cron job that logs the speed with their cli client every 5 hours and I use it to keep my ISP mostly honest.

The resulting data can also be used to map peak network congestion so you don’t end up with network buffering issues when you are watching the latest episode on your favourite streaming service.