How do I verify the speed of my NIC

Determine the device name

From terminal determine the name of the NIC in question
The command ip a will list all Network Devices along with information on the network devices.

ip a

Example of using the ip command with the option a 

# ip a 
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 7c:f1:7e:b2:76:11 brd ff:ff:ff:ff:ff:ff
    inet 10.169.212.69/24 brd 10.169.212.255 scope global dynamic enp2s0
       valid_lft 54188sec preferred_lft 54188sec

In the example above enp2s0 will be checked for network connected speed.

ethtool command

The ethtool command is used to display/change Ethernet adapter settings.

ethtool usage can be found here

Find Connected Network Speed

Use the following for our example. Change the network device name as needed.

ethtool enp2s0 | grep Speed

Example

# ethtool enp2s0 | grep Speed
        Speed: 2500Mb/s