I can only answer for myself, but will love to read others' replies: what are the red flags that make it so easy for an objective person to shout 'leave the fucker' ? Two things:-
1] That element of resentment; blaming; attacking their partner for, basically, being who they are.
A person who is honestly confused and sad, or even has an honest gripe about their partner, will tend to share it as gently as they can. Of course the partner will feel upset, anxious, and will wonder whether they should change. But this will happen in an atmosphere of mutual concern. There will be a shared feeling of "can we do anything about it?"
If the belittled partner feels - well, belittled and got-at - the other is, like it or not, being abusive toward them.
2] Dismission & 'absence'. With plain old abuse, the bully is engaged with their target. They'll harangue them, pick their words apart, follow them round the house, block their path, use passionate language, etc.
Unfaithful bullies do this too, of course, but even so their partner will notice an increased distance. Maybe the verbal attack or thump will be followed by silence or the bully walking away. Maybe they'll take to leaving the house after an insult instead of staying for the argument. When you live with someone you notice subtle alterations in their attitude.
And there is always mentionitis in the early stages!