They can't in any large scale way. Detecting and tracking variants, essential for public health efforts, requires complete nucleic acid sequencing of the virus. Currently, only specialized labs can do this, on a small sampling of positive COVID-19 swabs.
They test a few swabs and then make up numbers to report the percentage of the variant over all the tests. Stating that 9% of a group have this new and improved variant when in reality they may have sent .01% of swabs to a lab that specializes in the variant testing is pushing any kind of believability.
I would imagine it’s expensive and time consuming too? Maybe they are doing it, but seems improbable that thye are sending out samples from all over the US to be tested in special labs?