Windows 7 - Determine if a Product Key is already in use on another machine?

I have multiple users, all remote and I have another machine I need to load with Windows 7, I know one of the retail versions of windows 7 I have has yet to be used, just not sure which one...

Other than walking all users through the process of checking product codes to find which one is not in use I was hoping there is a way online where I can enter the product keys and it will tell me which have been activated and which have not been?

Anyway to do that?

I know I can install one and try to activate it, but I also know if I format and reload a machine it will let me activate that version of windows as many times as I want on that machine to so it's not going to tell me if it's in use or not right away, instead it will just hose one or both machines if it is in use already - not a great solution.

You can install auditing software in a server to query such data from machines on a network - but other than that, there's no alternative to checking each machine using either MGADiag (which has the added benefit of checking the license status) or a KeyFinder such as Magical JellyBean or Belarc Advisor.

One other possibility is to phone the MS Activation Center and request their assistance - but I doubt that you'd get an opinion on more than one or two licenses during each phone call.