Social graph metadata. The GGP said they would all be "talking to each other."
I think by 'detect' they mean 'detect by people who might fall for it' not 'detect by national security agencies.'
I don't see any way for the average social media user to detect whether they're engaging with a botfarm unless the social media company exposes their analytics, and they don't, and they aren't likely to start unless forced.
I actually was thinking the people who'd do the detecting would be the social media site operator (e.g. Twitter), who would then shut down the inauthentic activity.