I give anyone three chances, three sessions. After that, I know. Three is time enough for weirdness to resolve itself, but I also tell the person what they're doing that's making me uncomfortable whenever I can. Sometimes I can't say anything, and that is the biggest clue: if I am not comfortable enough to say, "When you do X, I feel Y," then it's not a good situation, because some sorts of roles are developing, and they're not healthy ones. What they ARE, is harder to say. They're just not the right foundation for therapy.