Hmm, I mark them for a few reasons, and now I have recently changed my marking scheme
As my story isn't all about the smut, I put hearts in the title saying there is smut in the chapter and approximately how much by the number of hearts.
Usually, I will have an A/N at the top of the chapter saying where the R-18 is and approx number of words it is.
Then I usually put a line break in before the smut begins, saying R-18 below.
I have now added a fetish tag in a spoiler tab at the line break, so if a user doesn't want to read that particular fetish, they can skip over it. Such as Inflation - Breasts or Light Femdom - Explain the scene in a short sentence. I have already gotten positive feedback on this. Obviously, if there is no actual fetish going on, I don't add anything.
Then, if the story continues after the R-18, I have another line break saying End R-18
Also, if there is any story within the R-18 that might be critical, I have been summarizing it in a spoiler tab at the bottom of the chapter.
I really don't think this is too much if your 'Smut' is actually more like a light novel that doesn't fade to black every time your MC gets laid.
I honestly started to do this for two reasons. I was reading another novel where the MC ended up doing a r*pe scene out of the blue... thankfully, the author had given a warning on this with a properly tagged chapter. If it wasn't, I would have dropped the novel like a hot potato, tags on the main page or not.
The other reason is that I detest some fetishes that some people seem to add to their writing, but don't necessarily need to be tagged. Like anything to do with feet. I would rather read about a damn r*pe scene than read about a foot job. It is just disgusting to me.
Anyways, thanks for coming to my TED talk, I have an R-18 chapter to finish. Make sure you tag what you're writing responsibly!