Agree with PP - body language is big. Another big telling sign is how well (or in how much detail) they answer your questions (tell me more about the position/your team/your company etc). Those usually come towards the end of the interview, and by then the interviwer has a good idea whether you're a viable candidate, and will not want to spend a lot of time answering these if you are not going to be continuing with the process.

Usually I just have a feeling about how it went based on the duration of conversation, whether the interviewer asked follow on questions or rushed through/seemed to cut things short, body language, etc. I used to be on the hiring team at my old firm and I could always tell when my manager wasn't interested in a candidate because his body language would change during the interview and he'd get really antsy when they were talking.

How can you tell if an interview went well? Other than getting the job or being asked for references of course.