There were dozens of hints we missed that Maytag and Bern were in a relationship (ok, so not everyone missed them. A good many had guessed by the time they had been revealed). So I'm simply wondering what hints we've missed from Bern that suggests she knew about Maytag cheating on her.
It's possible that Maytag may have told her about how she broke up with Xenfa and Bern simply guessed that the same was going on between them, but was able to tolerate it while she remained discrete. (Sorry for those who don't own the books and have no clue who Xenfa is.
She sort of referred to him here). But looking at the mention that Bern broke up with Clairen because of cheating
here you'd think she wouldn't be that tolerant (although they didn't specify that it was Clairen who cheated, but it's a good bet). There's
this little speech to Orrasong that may have hinted at her having developed a greater understanding for differences.
But without going into Book 0 for hints I think
this sequence where she tells Crest the story of how they met is our best bet. Right there, Maytag tells her exactly how it's going to happen, how she'll be fooled into thinking Maytag's wonderful but eventually believer, like everyone else, that she's nothing but scum... And Bern accepts it and pushes ahead anyway.