How do you know how well you did in an interview?

I had an interview the other week and I thought it went really, really well. A few times the director in the interview even said that my answers were well thought out, I had a great solution process, I was super put-together, etc. I know my education is good, my school has the top program for my line of work, I had prepared very thoroughly for the interview, and I had done most of the job responsibilities before at my internships with note-worthy results. However, I just got the e-mail today that I didn't get it and was pretty crest-fallen. Did I misread the signs? How do you know if you did well in an interview?

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