As others have said, it's a lot easier to be sure a T is not "the one." I'm still kind of feeling my T out after maybe 6-8 sessions, but I think he's going to be the one and I've made myself vulnerable/given him the power to hurt me, which I've never done with a therapist this way before.
I think things started to click around the 3rd or 4th session. Maybe he started to let some of his personality show, or maybe I was just able to start recognizing it, but either way it felt more personal and more human. I was able to start trusting him once I felt like I could "get" him to some degree.
I also had some idea what qualities I was looking for (and what qualities I was avoiding) because I've seen a lot of therapists in my life.