[angott]

You could technically do this, but such a technology is too expensive to incorporate in civilian use receivers that have to retail for a couple bucks.

Military receivers used by the USA and NATO allies can easily detect spoofing because they listen for signals on separate frequencies reserved for military use, with higher precision. On these frequencies, all traffic is encrypted using a private key that only the DoD has access to (in theory). In this case, it is easy to detect spoofing because your enemy cannot encrypt signals using the DoD's private key (they just don't have it). If the receiver is unable to decrypt the incoming signal (key mismatch), it knows there is something fishy going on. I would also speculate there are additional countermeasures which are not publicly available.

[gsich]

Just jam the other frequencies.