For PC desktops with Windows 7 (it’s something very similar for the other Windows versions like XP, Vista or 8), go to Control Panel > Network and Internet > Network and Sharing Center > click on Local Area Network. You should see there “Speed”.
In most cases, you will see “100 Mbps”.
For laptops that are working from a Wi-Fi connection.
Control Panel > Network and Internet > Network and Sharing Center > click on Wireless Network Connection (name of the Wi-Fi connection). You should see there “Speed”.
For routers, most of them have the speed marked on their back side. If you see 150 or 300, it means that it has 100 Mb/s LAN (Ethernet) speed and 150 or 300 Wi-Fi speed.
If you have a 200-1000 Mb/s Internet connection, you will need a 10/100/1000 LAN (Ethernet) router, which are pretty expensive. And a LAN card with 1000 Mb/s.