Detecting Spoofed MAC

[divinepresence () gmail com]

Hi all
Is there a tool to determine whether the MAC has been spoofed on a system (Win/*nix) for a given interface? Also, is it 
possible to know the real MAC in such a case? I was wondering if you could hook up to some system info API which would 
provide you with this information assuming that this detail is stored at some location which is not affected by 
spoofing.

Thanks
Ankur Jindal