How do I know that this picture (taken at a recent magazine shoot) was taken at f/2.8?

Given the lens (a 16-35mm f/2.8 lens), I look at the shape of the out-of-focus lights in the background. If the aperture was stopped down, they would be hexagonal or octagonal. Round out-of-focus spots means the lens was wide open: f/2.8, therefore.