Spoiler culture has got strict enough that many readers now say nothing at all, which is its own loss. There's a middle path, and it mostly comes down to leading with your experience rather than the plot.
Ask where they are first
One question — "how far are you?" — solves most of it. It costs nothing and instantly tells you what's safe. Do it before you say anything, not after you've already started a sentence you can't finish.
Talk about feeling, not events
"There's a bit in part two that wrecked me" gives someone everything they want — anticipation, connection, a reason to keep going — and reveals nothing. "I didn't trust that character from the start" is a real opinion that spoils nothing. The emotional layer is almost always spoiler-free.
Say what it did to you. Don't say what happened.
Know what actually counts as a spoiler
Genre, tone, pacing and whether you liked it: never spoilers. Character deaths, twists, endings, and whether a relationship works out: always. The grey area is structure — "it changes completely at the halfway point" is usually welcome, but if in doubt, ask.
Flag it, then leave a gap
In writing, say "spoilers for the ending" and put a few blank lines after it, so nothing is visible in a preview. In speech, ask before you continue. The point isn't ritual — it's giving the other person a real chance to opt out.
The better fix: read at the same time
Every spoiler problem disappears when both people are on the same chapter. That's the case for buddy reads and book clubs — see online book clubs: how to join or start one.
It's also the idea behind Arwy: as you read, it connects you with people currently on the same book, and inline comments sit on the exact line — so you only see notes for passages you've already reached. Nobody has to police anything.
For what to actually say once you've found someone, these conversation starters are a good place to begin.
Talk to people at your exact chapter. Get Arwy on Google Play.