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

No smile on the face of the interviewer and no personal platitudes when leaving the office.

When I know I was overly nervous the whole time.

If you have a common sense, and prepared your interview, then you would know if you've answered all the questions and knew what they were looking for. You can tell by the look of their facial expression, tones, and eyes movements, and hand gestures tell you many things about their responses as well. If you've done your best then it is what it is. You not getting the job doesn't mean you didn't do a good job in an interview. In fact even if you did really good in an interview doesn't mean you are getting the job. There could always someone better than you with less qualification that they might just like. Or if they are hiring a relative or through a referral, they likely have more chance than you. You can ask them directly you did in your interview.

You get a polite "thanks for coming in".

When the interviewer does not want to engage in a conversation.

If you don’t leave an interview feeling confident, you’ve done bad.

If they said the interview will be with multiple people and then after the first person, they say you are done. It means they don't want to waste their boss's time meeting you.

There are no follow up questions.

Facial expressions and body language say it all

When you can’t answer relevant questions

When I leave and didn’t feel challenged.

They don’t call back.