ethtool command is helpful to check the speed and change settings of network interface, also get information which works now for most network interface cards.

How to Install ethtool on Linux

Ubuntu / Debian:

 # sudo apt-get install ethtool -y

CentOS / Red Hat Enterprise :
 # yum install ethtool  -y

Package      Arch        Version        Repository   Size
ethtool      x86_64      2:3.15-2.el7   base         113 k
  Installing : 2:ethtool-3.15-2.el7.x86_64                                                                                                        
  Verifying  : 2:ethtool-3.15-2.el7.x86_64 
To check the network interface of eth0 run,
#  ethtool eth0

Settings for eth0:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full 
                            100baseT/Half 100baseT/Full 
    Advertised pause frame use: No
    Advertised auto-negotiation: Yes
    Speed: 100Mb/s
    Duplex: Half
    Port: Twisted Pair
    PHYAD: 2
    Transceiver: internal
    Auto-negotiation: on
    MDI-X: on
    Supports Wake-on: pumbg
    Wake-on: g
    Current message level: 0x00000007 (7)
    Link detected: yes

Change the Network Speed to 100MBps full Duplex mode, run
# ethtool -s eth0 autoneg off speed 100 duplex full

Change the network speed to 1000Mbps, full duplex mode, run
# ethtool -s eth0 speed 1000 duplex full autoneg off

To make a speed change permanent for eth0, we need to set or add the ETHTOOL_OPT environment variable in network configuration file 
# vim  /etc/sysconfig/network-scripts/ifcfg-eth0
ETHTOOL_OPTS="speed 1000 duplex full autoneg off"