For comics: I will mark an episode mature if there's any nudity where genitalia or female nipples are distinguishable. And if there's any censored sex. Per the content guidelines.
Gore is trickier. I don't draw much of it, but I would mark it mature if it's highly detailed and leans more toward the realistic side of things.
I will also mark an episode mature if it contains slurs, you know, ones that target specific minority groups.
For novels: I will only mark for explicit sex scenes. I'm not as worried about gore since it's up to the reader to imagine how visceral it is. I will mark it for instances of phobic content too if it contains slur usage, just as I do with comics, but that hasn't come up yet.
For other content, I would just put up a content warning at the top of the episode. Or if it's featured prevalently in the series, I would have a disclaimer before the first episode.