How do you check the current duplex value of a network card set to Auto negotiate in Windows?

At command prompt:

powershell "Get-NetAdapter | SELECT name, fullduplex | where name -eq 'card name'"

Enjoy.

Without looking at the switch, the only way it seems possible is if the windows driver for your NIC reports this information. To see if it does, go to Control Panel --> Administrator Tools --> Event Viewer. You then want to look at the System logs (in Windows7 this is under the Windows Logs tree). Once you found the System logs, click on Source at the top to use as the sorting criteria. Now look for your NIC driver, mine for example is b57nd60a. Scroll through all the entries that your NIC driver has made and if you're lucky you'll see what speed it negotiated at in the event report.

Type the following into command prompt:

wmic NIC where NetEnabled=true get Name, Speed

It should tell you.