GPS is not typically used to confirm a position that is known accurately by other means, and that is not its purpose. Only in those cases where there is a manifest conflict with independent spatial information will the problem be evident.
I am not so sure about that. The most common use of GPS is in satnav in cars. Satnavs typically show a map, and typically it is very easy to confirm your position on a map. Any inaccuracy by more than the usual few meters would be quickly noticed by the majority of GPS users.
* Air and sea port operators and navigators
* Military personal running supply lines
* Military personal on foot in operations and training
* Space-X
* NASA
* River boats
* Fresh water fishermen
* Etc
Out of all the possible users who would notice a 300m deviation just based on visual reconciliation, I personally would not say it would be so rare that the USAF would not find out very quickly. Of course, this is ignoring the equipment that would likely detect the issue way before somebody in the Army started phoning the USAF.
There's not so many GPS satellites out there that you're going to be bouncing around them all the time - even if only one is affected, it would be very noticeable for extended periods of time.