His papers present things to look for in the current cosmic microwave background that would be evidence. In other words, data we have access to.

He's made some claims that these "circles" have been found, as some colleagues of his are working on making such observations. Those "circles" have not been independently verified, however, nor is it yet possible to rule out process error in the data, or the observations calculated from the data.

The CMB should show cold spots from evaporated black holes that have expanded to several arc-seconds in diameter. It requires very sensitive survey data, though.