cat

$ cat /sys/class/net/vnet0/speed
100

ethtool

$ sudo ethtool vnet0
Settings for vnet0:
    Supported ports: [ TP ]
    Supported link modes:   10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Supported pause frame use: No
    Supports auto-negotiation: Yes
    Advertised link modes:  10baseT/Half 10baseT/Full
                            100baseT/Half 100baseT/Full
                            1000baseT/Full
    Advertised pause frame use: No
    Advertised auto-negotiation: Yes
    Speed: 100Mb/s
    Duplex: Full
    Port: Twisted Pair
    PHYAD: 1
    Transceiver: internal
    Auto-negotiation: on
    MDI-X: off (auto)
    Link detected: yes

lshw

$ sudo lshw -class network
  *-network
       description: Ethernet interface
       product: Ethernet Connection (2) I219-V
       vendor: Intel Corporation
       physical id: 1f.6
       bus info: pci@0000:00:1f.6
       logical name: enp0s31f6
       version: 00
       serial: 1c:1b:0d:e3:b7:8c
       size: 100Mbit/s
       capacity: 1Gbit/s
       width: 32 bits
       clock: 33MHz
       capabilities: pm msi bus_master cap_list ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
       configuration: autonegotiation=on broadcast=yes driver=e1000e driverversion=3.2.6-k duplex=full firmware=0.2-4 latency=0 link=yes multicast=yes port=twisted pair speed=100Mbit/s
       resources: irq:131 memory:f7200000-f721ffff
  *-network
       description: Ethernet interface
       physical id: 2
       logical name: vnet0
       serial: 1c:1b:0d:e3:b7:8f
       size: 100Mbit/s
       capacity: 1Gbit/s
       capabilities: ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
       configuration: autonegotiation=on broadcast=yes driver=macvlan driverversion=0.1 duplex=full ip=10.239.82.126 link=yes multicast=yes port=twisted pair speed=100Mbit/s