I was somewhat comfortable from the first time I met my T. She just puts out this comfortable energy overall, and it automatically made me feel at ease.
Fast forward: the thing that ultimately made me feel 100% comfortable with her was when I told her about my SI. I know that seems odd, but just the way she handled it, and the way that turned out is what ultimately let me know that I could trust her with anything. To me, I think that was the make-or-break moment in therapy, and needless to say, that's what I feel like ultimately "made" my therapy.