And they know that how exactly? My wife had an upper respiratory infection for a few weeks. Tested negative for Covid multiple times. I asked the ER Doc about the “Delta” variant and he laughed and said he had “no idea where they are coming up with these numbers, because there is no separate test for Delta” He explained that you test either positive or negative for Covid. There’s no way to know what variant you have.
It’s absolutely possible to detect the Delta variant. The type of test used is called genomic sequencing, which is a different test than regular COVID tests. States send samples for the second test - genomic sequencing - to specialized labs every day. That’s how they detect new variants, and how they can tell which variants are more prevalent at any given time. Here’s an article describing how they do it in Minnesota: