How to tell if you are in a failing relationship?
This is such a tough question. It's not always apparent, for one.
The best way I can describe it is when you can easily see yourself living without the person. It might even make you feel happy to think that way.
I'm not saying anyone should run out and leave their significant other if they feel this way. Just because a relationship is failing doesn't mean it's beyond salvageable.
That said, every person should have personal boundaries that are a hill to die on. If someone hits me, my children or my pets then that relationship hasn't only failed, but there's no way it can be redeemed.