By using PowerShell like this:
Get-WmiObject -class Win32_PerfRawData_Tcpip_NetworkInterface -ComputerName Localhost | Format-Table Name,@{label="LinkSpeed(MB)";Expression={$_.CurrentBandwidth/1000000 -as [int]}}
which will return this: 
I first learnt how to do this by reading this blog post by Dustin Hedges.
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Dale