Same way that they smell witches out in sub-Saharan Africa. If you are a witch-smeller, and you say you smell witches, and that so-and-so is a witch, the crowd just tears them apart.
Same thing with Omicron in the sewer. A man in a white coat announces that he’s examined the sewage and found Omicron in it. Then they shut everything down, make everyone wear masks, and shut all the schools and the “non-essential” businesses down. He’s got a white coat and a diploma, so you know he knows more than you, and submit. You wouldn’t want to feel muscle ache and fatigue, would you.