Determine / Find ethernet connection speed

You need to use ethtool command todisplay or change Ethernet card settings. It is used for querying settings of an Ethernet device such as
=> Link speed
=> Autonegotiation
=> Coalescing settings
=> Change the rx/tx ring parameters
=> Setup new speed and much more
Most intelligent network devices use an autonegotiation protocol to communicate what media technologies they support, and then select the
fastest mutually supported media technology.

How do I determine Ethernet connection speed?

Type the following command to get speed for eth0:

$ ethtool eth0 | less

OR
 
$ ethtool eth0 | grep -i speed