Character death: I don’t give warnings for this. Sorry. It’s one of those things that are usually in my story for shock value (not merely shock value, but yes, I do want my readers to jump a bit when I bump off Palpatine).
Slash or Het: I usually list the pairings in the story, or enough of the characters that it’s easy enough to guess the pairings.
Violence and traumatic non-sexual events: The violent stories I remember off the top of my head (Shmi’s Stories; Through A Mirror Darkly), I warned would be gory. And I usually put ‘dark’ or ‘violence’ if there’s any indiscrete bloodletting.
Rape: I’ve only ever written one story that this happens and I didn’t warn about this. Most of my readers didn’t even know this had happened until long after the event itself had occurred. I felt guilty about that at the time because I know I would want to be warned for rape, especially between those two characters. But I honestly don’t know how I would have written that without giving the whole story away.
Underage sex: I’ve never written this and I’m never likely to write this, but I would definitely warn for this.
Incest: The one incest fic I did write, I didn’t warn for. But nobody seems to have figured out that it was an incest fic either so I think that’s about fair.