The interesting thing is when machines will write something that feels more human than anything humanity could write. I believe that will happen sooner than people think.
Made me think of:
https://www.youtube.com/watch?v=OyO18QrJ3zw (White Zombie - More Human Than Human, referencing PKD's "Do Androids Dream of Electric Sheep?")
Individuals checking individuals: yeah, you can detect non-intelligence. But if I told you half of all buzzfeed headlines are machine-written... would you believe it?
Honestly, yeah. Buzzfeed headlines aren't exactly the most creative or hmmm...deep?...meaningful?
There's only so many ways you can write
'10 reasons why x does y and that's bad'
Or
'Omfg x just happened, better y right now.'
Or it could just be a fluke random AI achievement. It will still be humans judging it as "more human than anything humanity could write" or not.