I am using Ubuntu 16.04, and have setup a bridged network for KVM with the following configuration in /etc/network/interfaces:

auto lo
iface lo inet loopback

auto eno1
iface eno1 inet manual

auto samba
iface samba inet dhcp
bridge_ports eno1
bridge_stp off
bridge_fd 0
bridge_maxwait 0

--------
#cat /sys/class/net/eno1/speed
100
#cat /sys/class/net/samba/speed
Invalid argument

#ethtool samba
Settings for samba:
Link detected: yes

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So in most cases I get no link speed data at all. However when using tools such as FusionInventory, the bridge and all other virtual servers are listed with a link speed of 10Mbit/s. I am kind of guessing this is just a fallback value, as my Debian Stretch guest system list the following data for ens3:

ethtool ens3
Settings for ens3:
Supported ports: [ ]
Supported link modes: Not reported
Supported pause frame use: No
Supports auto-negotiation: No
Advertised link modes: Not reported
Advertised pause frame use: No
Advertised auto-negotiation: No
Speed: Unknown!
Duplex: Unknown! (255)
Port: Other
PHYAD: 0
Transceiver: internal
Auto-negotiation: off
Link detected: yes

Most network adapters will negotiate one of: 10/100/1000 Mb/s. Usually syslog will tell you what the connection speed is.

One of the ways to speed test road vehicles is to do a flying kilometre - measure it over a distance.

Why not set up the computer equivalent? Get your fastest source and download 1G from it. As long as you're not bridging multiple gigabit nics there shouldn't be a problem if you can feed it with a gigabit nic.