The free network speed testing tool iPerf3 can benchmark the bandwidth.
You need two machines for testing.
On one machine ("server") the tool is started in server mode:
From another machine ("client") you can then measure the bandwidth, with server being the IP or name of the "server". The "client" must be able to connect to the corresponding port of the "server" (e.g. iperf3 default port 5201):
iperf3 -c server
Although you can measure against public servers listed at https://iperf.fr/iperf-servers.php, this will most likely not show you the maximum bandwidth of your setup. Anyway an example would be:
iperf3 -c bouygues.iperf.fr