So heres my current list of things that tip me off when I see them all at the same time. I bolded that to emphasize that the use of a couple of them does not mean necessarily you aren't human.
1- em dashes, obviously.
2- ellipses. I think slowly, the use of “…” is replacing em dashes. They will always have exactly three dots, no more, no less.
3- perfect grammar and syntax, no typos, no abbreviations, no slang. Obviously you can write properly, I encourage it, but come on. This isn’t an exam.
4- the overuse of quotation marks when referencing what someone said, or when paraphrasing an emotion. Humans just don’t really do that unless the quote itself was particularly shocking and they can’t get over it.
5- perfect structure, with a set up, context, the conflict, the internal struggle and feelings, and the question. Most people won't structure their problem that logically, because emotion is guiding their writing.
6- TL;DR, with a semi colon in between specifically.
7- dramatic turns of phrase.
Any others?