I just got off the phone with Sawyer. I asked her about how to tell if your sawyer got frozen? I mentioned the turkey pop up kind of thing, and the colored strip in your phone to tell if it got wet. She said any kind of frozen sensor would be crazy expensive.
I also asked her if blowing out the filter in below freezing weather might help. She said that would be even worse. If the filter was full and froze there's a chance both sides of the membrane would freeze and when thawed just might remain intact.
If one side of the membrane has water and the other air, there's no chance the membrane would remain intact.
The Sawyer uses the same stuff as dialysis filters.
FYI