Personally, I have no clue, but the number of independent groups and governments who came to the same conclusion gives me reasonable confidence that the publicized progression of mutations are reliable. (We must keep aware that this type of assumption is vulnerable to a Sybil attack of information sources colluding to provide false information and censoring information that questions or contradicts it.)

Many healthcare groups and governments monitored DNA samples from their infected citizens as the Covid pandemic progressed. The assumption is that infection samples, were compared with prior samples as well as established Covid DNA sequences. This would make it easy to, not only associate by the timing of the infection during the Covid pandemic, but also do specific DNA comparisons of the strain; One would expect many DNA similarities. Many studies analyzed the prevalence, timing, and geographical reach of new strains as they emerged, as well as studying the DNA differences and similarities in depth.