This is a bit of the interesting aspect to this failure. Official channels have overall been pretty weak on details on what all happened in this error. Most public knowledge of what happened has been gleaned from the signals coming off the satellites themselves which anyone can see. This live data combined with understandings of how the system should work is most of the public knowledge of what has been going on.

I don't have a ton of knowledge into the deep technical aspects to GPS, but I imagine we would probably have some similar clues of an outage of this scale in GPS. Maybe a little less technical details of what is happening behind the scenes, but knowing a high percentage of satellites entering a no guarantee precision mode should be possible.