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.
One of my current heuristics is the expression of weirdness. I can see it in the way I write, there's like this thread of a human processing information through emotional and idiosyncratic experiences. There's a certain cadence to the way thoughts come out. Maybe it can be emulated eventually, but that might be farther off.
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?