The indicator, imo, is the long established presence of plague bacteria still occasionally found in mice, rats and suchlike in the western and south western US, but not so much in the eastern parts of the country.
I mean, that’d seem to be an indicator if I’m correct in my understanding that the plague originated in the Asia AO.
I have read that bubonic plague is endemic among wild rodent populations in 17 Western states. Have any genetic studies ever been done to see what the similarities might be to Asian forms of bubonic plague versus the European varieties? They would probably have to dig up old remains to study the types that existed historically. I also wonder if BP could have been a factor in disappearance of Anasazi and other tribal groups? Any studies of causes of death among the buried?