This is a tough one that I've been pondering, too. I think given the graphic content of my comic, the kind thing to do would be to put trigger warnings at the beginning. However I'm thinking they should be vague. Instead of saying explicitly, for example, that a character has his arm ripped of mercilessly, I'll just put in a general "Graphic violence" or a blood warning. Instead of saying one character uses such and such bad word, I'll but a "Language" warning. I think this is something you would probably be able to do without giving out spoilers. Just be vague. Some other general warning labels artist could use might be, "Drug Reference", "Strong Sexual Content", or an extra general "Mature Themes". More specific examples but still not quite spoilers labels could include, "Instances of Domestic Abuse", or "Graphic Nudity".
You could also categorize your comic in the same fashion as established film or comic age guidelines to give the readers a clue about what to expect. For example, my comic would fall under an "OT" rating as established for published manga. "OT" is classified as manga that "may contain more explicit sexual situations and some violent scenes, including blood and gore." Or, use movie guidelines such as "PG-13", "R", etc.
I think using generalized terms or rating system will help readers evaluate whether the content is safe for them to read, without potentially spoiling major plot points for themselves.