If they say something like " Now how can we convince you to come here?" as the interview's ending, it's pretty good sign. Or if the interviewer is extremely solicitous and helpful about helping you find your way to wherever you're supposed to be next, saying things like "We wouldn't want to lose you..."
But just going by the feeling you have is pretty unreliable. I got some rejects and waitlists from interviews I thought had gone fantastically last year. Remember, coolwaters, the medschool interviewer is not making the decision alone, but only giving input to the adcom that makes the final decision based on the interview plus multiple other factors.