https://serverfault.com/questions/207474/how-do-i-verify-the-speed-of-my-nic

Thanks to serverfault I found how to check link speed between two debian machines – iperf3

You can see the expected max speed using ethtool:

# assuming eth0 is the interface we care about
~$ sudo ethtool eth0 | grep Speed
        Speed: 10000Mb/s

To see real-time speeds install iperf3, then run one machine as server and one as client:

# on both machines
apt install iperf3 -y
# say no to the install at startup - this is just for occasional testing

# on machine A, which is 192.168.1.3
iperf3 -s

# on machine B - reach out to machine A's IP address
iperf3 -c 192.168.1.3

I got 250 Mb/s whilst doing a backup on the cabling I installed myself 25 years ago (before gigabit was a thing) to a USB ethernet 25m away from the switch, so I will take that.