I actually struggle with this a bit. I tend to assume positive intent, which means I suffer fools, and sometimes assholes. It's probably as simple as setting boundaries and standing up for myself, but anyone have more helpful suggestions?
It's not your job to find the good person beneath the layers of trash. Generally speaking, our faults don't dwindle upon closer inspection, and even if they did, life is short, and you probably have other things to do.
If this is someone that you encounter a lot, there is nothing wrong with being wary and observing their trends.
You don't always have to have boundaries, but it's okay to sometimes decide that you have some. If you try to define your boundaries perfectly so they can never be argued with, that's a trap to be exploited by those who like arguing.
I have the same problem. What I've found to be helpful is that I have several friends who don't gleefully suffer fools and are always willing to help serve as a second point-of-view on whether someone is just generally an asshole.
It's a lot easier to tell if someone is trying to sell you a snow-job if you can get the perspective on how they are outside of your field-of-view.
If I assume you intend to make me a delicious soup, that is still quite dangerous to me if your intent is to make me a delicious shrimp gumbo -- You do not generally intend me harm, but to achieve that intent we need to communicate about my specific allergy to shrimp.