Plan to arrive 15 minutes early. If you're going to be late, call. The manager will check to see if you're reliable. Notice the interviewer's body language. If the manager is pleased with what he or she is hearing from you, they'll lean forward in their chair, look you in the eye, smile, and give other kinds of friendly and open gestures.
Time is also a good indicator.
If your interview lasts longer than an hour, it's a good sign. A thank-you note serves several purposes. First, it shows that you're civilized and that you truly appreciate the opportunity to be interviewed. It also serves to remind the interviewer, several days after the interview, that you are a candidate for the position, and that you still want the job. It also gives you an opportunity to mention any key points that you may have forgotten to bring up during the interview.