It's so easy for me to tell when I didn't get the job. Sometimes you just know this. It could be a look that the interviewer gives you just as you walk in or a look that you get when you answer a question.
I've also noticed that if an interviewer rushes you through and spends 5 - 10 minutes with you that is also a good indication that I didn't get the job. I think if they were really interested they would give you at least a half hour.