This is a simple command that will allow you to check the link speed of your Windows boxes using powershell, just replace localhost with the name of the machine you want.
Get-WmiObject -ComputerName 'localhost' -Class Win32_NetworkAdapter | `Where-Object { $_.Speed -ne $null -and $_.MACAddress -ne $null } | `Format-Table -Property SystemName,Name,NetConnectionID,Speed
This is measured in bytes so if you don’t feel like counting zeros you can just dump the output into a converter found here