This seems like a terribly difficult problem. I'm sure today there are some give-away signatures, but I put my money on the impersonators in the long run.
In my fantastical imagination of that implementation, I would simulate "sim agents" as they live a life, collecting unique experiences that are the result of interactions with other sims. They then build up personalities, which influence their expressions in other mediums that real humans would consume. It's psychotic and possibly evil and I thought of it first, so no one better take my idea.
You might be able to see what I mean just in this comment itself. Could this have been written by an AI?