What are the common signs that a graduate interview has been a success or a flop

I am not familiar with the graduate interview process. Are there signs during and after the interview that student will be able to tell whether he or she will be accepted or rejected in into the research group?

More importantly, how would an applicant know if the graduate interview was a flop. For example, if the professor doesn't follow up with a letter after the interview, is that a sign that the student will probably not considered to be a part of the group?