Interesting thoughts.

I have had a similar question about marriage.
Like, if you and your spouse are challenged if that that lady is actually your wife of 20+ years, how can you prove it?
Nobody carries their marriage certificate in their wallet.
Even so, it's just a piece of paper with a couple of names on it. It's not even notarized.
If you could even find that paper anymore, after decades and many moves.
Of course, the wife might know where that paper is. ;-)

There are some heuristics to figure if somebody is most likely a US citizen. Just a quick-and-dirty triage, probably is vs. might be not.
Speaking accent. There are 2 or 3 or 4 accents that are very common.
Depending on age: Where were you when JFK was shot? Where were you on 9/11?
No longer a good question: Where were you on Dec 7?