I know that for the most part you should stay out of your partner's relationships. I am trying hard to do that, although my partner tends to drag me into them. Quick backstory - we've been together a long time and are best friends, so we have the bad habit of using each other as a sounding board for our other relationships.
Anyway, what I'm wondering is if there are ever any red flags or warning signs that a person is trying to sabotage your relationship. I have a very bad feeling about the person that my partner is dating right now, but he gets very defensive and upset when I bring it up. I've tried talking to her, but I feel like she's very dishonest.
Are there ever any serious signs that someone is not to be trusted? Anything that I can do if I feel like this is a potentially toxic relationship? Or do I just have to grit my teeth and wait it out?