Good question and one I have pondered much.
One thing that comes to mind is for me to ask myself how I feel around a particular person. My roommate is a huge N. When I interviewed her I recall thinking to myself, as she was leaving the interview, "wow! that is just about the kindest, most empathetic, warm hearted, could not hurt a fly kinda person that I want to live with." Her N tendencies were well covered over in her covert mask of sanity. It was after a month of living with her, being "slow cooked," as you say, her daily inconsiderate and very selfish ways that at first I just kept making kind excuses for her, telling myself well she must just be overwhelmed with moving and needs time to adjust and get to know me my and the place. She seemed to show very little common sense about living with others. But, then, after her third dinner party less than three weeks, her friends were starting to become permanent fixtures and I could not even use my living room on a weekend, I said something to her about her behavior. She gave me the silent treatment for 2 months and tried to turn my other roommate against me.
It has been months now and things have worked out OK between us. I made mistakes, as is the case when not knowing someone well enough to know how to cope.
Looking back I can see now that she was projecting that kind mask or rather manipulating me into THINKING something about her rather than feeling my gut out, manipulators are good at putting thoughts into your head, overriding your gut. So I guess I learned to listen more carefully to my gut when meeting people and question, lovingly more where they are coming from rather than just blindly accepting them at first meeting as a kind person, I have always projected parts of myself onto others----need to stop doing that.
Deep down I want to believe that ALL are good and not manipulating.