Find Ethernet Connection Speed

Type the following command to get speed for eth0:

$ ethtool eth0 | less

OR

$ ethtool eth0 | grep -i speed

You can also use the following two commands:

$ dmesg | grep eth0 | grep up
$ dmesg | grep bond0 | grep up

OR use the command line utility called mii-tool to checks or sets the status of a network interface’s Media Independent Interface (MII) unit:

$ mii-tool -v eth0

Src: https://www.cyberciti.biz/faq/howto-determine-ethernet-connection-speed/