Not very much. They knew that the city was massively damaged. They knew, from Japanese radio reports, that train service had stopped in the area. They did not know specifics about casualties. They learned more these sorts of things right after Nagasaki. The first reports from the Japanese with any real details about damage did not come out until August 9th, after the Nagasaki bomb had been dropped. (More on those here.)

Those nice aerial photos were are used to seeing were not created until well after both bombs were dropped. One of the first things the US did once it began occupying Japan was to do a damage survey of the two cities.