The interviewer is late or poorly prepared
If an interviewer is late or turns up unprepared then this is a sign that the company is poorly managed and doesn't value employee time. Chances are, if they can't be organised in the interview stage then this reflects the overall organisation of the company. It can also be a red flag if your interview is rescheduled numerous times. If the interviewer can't stick to an arranged time, this could show that there's poor time management throughout the company, or that people are drastically overworked.
The interviewer is vague
When someone is interviewing you for a job, they should know the ins and outs of the position that's advertised. Not only does this show that they're going to be a competent manager or boss, it shows that they care about finding the right person for the job. If they don't know much about the job you're interviewing for, or worse still, don't seem to care about your answers, this is definitely a red flag.
The interviewer is unable to answer your questions
Similarly, if you're asking questions about progression, work environment or any other aspect of the job that the interviewer cannot answer then this could be a very bad sign. You want to be able to make your decision about whether or not to take the job with all the information. If you walk away with unanswered questions then it could turn out that it wasn't the right job for you later down the line.
The descriptions of the work environment are neutral
You're going to be spending a lot of time at this new workplace if you're successful in your interview, so it's important you know what exactly you're getting yourself in for. If you ask about the work environment and the interviewer's answers are vague or neutral, this could be an indication that they're actually unhappy with the place they work. If they're positive and have plenty of good things to say, this will definitely put your mind at ease. If you're still unsure at the end of the interview, ask if you can have a quick walk around the workspace.
You can't get a word in edge-ways
If the majority of the interview is taken up by the interviewer talking at you, then this is definitely a red flag. Chances are, they're not particularly bothered about you, your background, skills or interests, they're more concerned with getting someone into the job quickly and without fuss. If you don't feel like you've had an opportunity to make yourself heard in an interview, it might be wise to keep looking for a different job where you'll be more valued.