I started reading that book. First of all, the author doesn't pull any punches with his examples. It can be triggering.

I can see why my therapist recommended it though. Her theory - and it's a good one - is that the people who abused us screwed up our internal monitoring. Our instinct about who was good to us, vs who was bad to us, got blown to bits. This book is all about recognizing our instincts about people, especially those who would do us wrong.

Firstly, if you are doing any of these things, then the person in question is not someone who is trustworthy, and if you think they are, you are in denial and ignoring your instincts while you try to explain why they deserve another chance:
rationalization - it's ok when you look at it this way
justification - this happened, which explains why they'll act that way
minimization - it's not so bad
excuse-making - it's not what it looks like
refusal - I don't care what the facts say, I'm going to stick by them anyway

The book also talks about how useless worry is. The example he used was, if you see that lava is moving toward you, you're not going to stand there and worry about what might happen if it reaches you, you're going to run away. Worry is a sign that you are ignoring your instinct. It evolves from the conflict between instinct and inaction. If you are worrying about something, then do something about it, in a nutshell. Either get to the bottom of it, realize there is no reason to worry, and calm yourself. Or, if there is a valid reason for concern, take action to correct for it so the end result will not be negative.

So the first concept, about denial, is this. If you constantly deny your instinct that someone is toxic for you, make excuses for them, etc, then you will lose the instinct. It will be come too muddled to be valid anymore. Honor your instinct and you will be able to tell who is trustworthy and who is not.

The second concept expands on that. If you don't worry about someone, but rather actively find out if your worries have any basis, then you will be able to know if they are trustworthy or not.