Sure, here's a few ways to cross-validate: If we had a high prevalence of Covid before there was any testing, we'd expect that the moment testing started, we'd

Sure, here's a few ways to cross-validate: If we had a high prevalence of Covid before there was any testing, we'd expect that the moment testing started, we'd see a high prevalence everywhere. We didn't. Most places saw only a small proportion of tests come back positive to start with, which then increased over time. (When looking at this kind of testing that's been done using antibody tests, it's good to remember that even a 99% specific test will mostly be returning false positives when the prevalence is very low). We can re-test historical samples of various kinds for Covid. That has been done, and while they can find individual cases from a month or two before the first confirmed "live" case, there's no sign of it being widely spread. We can also deduce the transmission patterns of the virus from the phylogenetic tree. If the virus had been widely circulating outside of China in 2019, sequencing would have show variants that are common in the west branch off from the Chinese ones earlier than they did.