Programmatically check whether Windows 7 is activated

The Software Licensing API - SLIAPI, take at look at SLIsGenuineLocal(). (Replaces LegitCheck)

You can run "Slmgr.vbs" on the machine and then redirect the output to check.

System.Diagnostics.Process.Start(@"cscript Slmgr.vbs > D:\\log.txt");

This will return all the information you need. Not the cleanest way to do it though.