Hard to tell isn't it? Well not really. As long as you know what to look for and when to look for it. In fact you could read your interviewer like a book if you keep reading. I am going to show you just a few signs to look for and respond to correctly. That way you can lead them to providing you with even more signs.
Sign #1 Time
How long the interview lasts can tell you a lot about how you are being received. A 20 minute or less interview for most industries means you are probably not getting the job. This might not be by any fault of your own. They may have another candidate in mind before their interview with you. Don't be disappointed just take it as a learning experience go through the motions and still send the thank you card (which most don't send).
Sign #2 Eye Contact
Are they looking at you or through you? The most common sign is that they look at their paper or stare into space while speaking to you. Also they are talking at you not to you. Like i said in sign #1 it may have nothing to do with you. All you can do is follow up and thank them.
These are the two easiest signs to spot thought there are others. The vocal tone, unprepared interviewer etc. Your next interview pay close attention to the way in which the interviewer speaks and uses eye contact. And if the interview ends in a short period of time just scratch that opportunity off of your list and move on.
Lastly don't be surprised if they follow up with another request for an interview. However don't sit there twiddling your thumbs neither. The important part to understand are the signs. Also it is important to know the difference between an interview that you handled badly vs one they conducted poorly.
Don't beat yourself up over it just move on to the next one. The better the signs the higher the probability of at least a second interview.