How do you know when you've done well in an interview?

Interviews are not mysteries. When they go well, the interviewer will tell you on the spot and likely schedule you for the next step right there and then. Having an open position is painful for all involved, especially the hiring manager, when they see someone they like - they don’t play hard to get.

I find it very important to establish an emotional connection with the interviewer. This can be from personal experience (for instance, familial or mutual friends), job-based or technical. For instance, it's good to take a question that moves you and to dig into it with the interviewer--get them to stake a position on it and challenge or reinforce them, in order to get behind the superficial question/answers. So even if I competently answer a laundry-list of questions, I consider the interview a failure unless I feel a connection has been made.

The feeling afterwards you can normally tell it was good or not

Guessing is all you can do.

You don't always know. Some interviewers won't give any indication of how it went.

You really cannot tell. You can ace an interview and still not get a job because there’s a better candidate. But in general, the interviewer’s body language and behavior can indicate your performance. Do they listen actively (nodding)? Is it more of a discussion than interrogation? look for microagressions - that’s a signal that it’s NOT going well.

When I know I've done my best (research beforehand, listened and not interrupted, had the opportunity to "sell" myself)! Not all interviewers are good so I don't always gauge how well I've done by the interview itself!

Look for all the interviewers to be engaged. If they seem detached, then they most likely don't care.

When you feel that everything went great and they want a second interview

I think there are several signs to look out for and they are dependent on the personality of the interviewer: 1). The interview runs over the time origianally scheduled. 2). Towards the end of the interview, the questions start to become less intense or business focused and more casual or even somewhat personal. This shows they are satisified with your responses and would like to get to know you better on a personal level. 3). Near the end, when they offer you an opportunity to ask some questions, their responses are well thoughtout and perhaps longer than usual. To me, this means they are investing their time with you rather than choosing to cut it short and get back to their routine. 4). When they are done, they ask if you have time to meet another person in the office who was not originally scheduled to meet you. 5). If they take the time to show you around the office after the interview to give you a visible view of the space.