I don't believe you can determine a date/time that it happened, but querying the age of the table is simple to do and then you can compare that with the freeze age parameters. A periodic manual vacuum freeze may preclude the system ever needing to perform the emergency autovacuum freeze, and as an added benefit, index only scans would be more likely to be chosen since the visibility map would be updated and the regular autovacuum runs would be faster since they can skip frozen pages. I think that applies back to 9.6 at least. It's possible it got implemented in 10.
My caffeine hasn't fully kicked in yet and I am still a bit junior on this list, but I would hope and expect senior people to correct me if I have misunderstood or misrepresented things.
