I guess the question comes down to how does one know if the battery pack is good? When I buy a regular used gas car, I can get all sorts of diagnostics about it out of the OBD2 port, pull a spark plug and stick a scope into the combustion chamber to see if there's any issues (e.g. on the walls).

With an electric car, how can one tell if the pack has been charged all the way up to 100% all the time (vs. the much better 40-70% range)?

This is the "term premium" of batteries it seems, and I honestly don't know if there's a reliable answer.