Windows 7: How to tell if Windows 7 has been activated

I have someone who wants to sell the full version on Window 7 Home Premium who said that he got it from somewhere that it was a returned product. The package was opened but nothing missing. How can you tell if the Activation Key has been used before. He also doesn't know if it is OEM or not. If it is OEM would it tell on the packaging and/or on the disk. The packaging looks like a retail box and not the cardboard envelope that an OEM usually comes in. He doesn't want to sell it if it isn't useful because of being activated before.

Thanks
ABG

As long as the key is only in use on one system at a time it should work.
You may have to use the phone option for activation.
As long as that specific key is not in use on another computer at this time it should work though.

As far as I know you can't tell until you actually try to activate windows with that particular product code.