[IAmGraydon]

Attempts to manipulate public perception are usually launched as campaigns with lots of content pushing the same message, often in different ways. In the case of mass media, for example, you could detect manipulation efforts by finding commonalities across multiple articles from one media outlet or by finding commonalities across multiple media outlets. It's possible you could distill this down to something like "From October 14 through October 23, content from media outlets X, Y, and Z seem to be correlated and are pushing the same message, and that message is ---".

Having that knowledge would be extremely powerful, effectively neutering manipulation efforts by identifying them as such. A person is far less susceptible to manipulation if they are aware of the belief that the manipulator is attempting to instill in their mind.