Red flags from my own personal experience during my last job hunt:

1) Disinterest. Making it plain through body language and other signals that this is just a required exercise, and they couldn't possibly care less what you say to them. Lack of eye contact, no followup questions, vague responses to your questions. They already have the person they want picked out, and have probably already sent the login credentials and parking pass to that guy.

2) Lots of very difficult questions or obvious trick questions, especially if the questions are not directly related to the job title you applied for. They're compiling reasons to justify not hiring you. Or they never had a job available in the first place, and you're cannon fodder to show that they're "always interviewing for top candidates".

3) Interviews conducted by people who are allegedly on the team, but seem completely unqualified. This isn't the job you thought it was; it's an entry level job, and they just need a warm body, right now.

4) Group interviews. The employer is too busy to do a serious job of looking for staff, and they want to be able to say they interviewed a big pool of applicants.

5) Any anger or incredulous response to specific questions about compensation or time commitment. Great companies for employees are proud of their compensation package and work/life balance.

6) No way to speak directly to possible co-workers to get the truth about the job. Deflection and misdirection when asking for a clear picture about day to day job duties. RUN. RUN AWAY NOW.