1.) Note their reaction when answering first few questions. Determine whether the person has the necessary knowledge about own company, offered job and whether he/ she has sufficient knowledge in your field to be able to "understand" your answers. Determine by watching closely the reaction to your answers whether the person is competent to hear and understand real world answers or whether he/ she is expecting a bunch of sales talk and wishes to be impressed.
2.) When being interviewed and person interviewing you is being obviously incompetent, you likely do not want to work for the company in question. If the person is tasked to be the "hiring face" of the company and is lacking the necessary skills, it is very likely that more incompetence is to follow.
Tip: Unless you wish to work for such people, you can spend rest of the interview asking questions and enjoy the answers. For example: "If, for whatever reason, it is impossible to complete a project, what actions do you take at your company to complete it?" In some cases, during of half an hour you can get enough material for a few episodes of Daily Show.
3.) Actually same as 2 - When applying for a senior position and person who is interviewing you is 25-30 years old HRM employee (and/or same age way down-the-"line-manager" that often means that "your resume looked good so let's see the person". It is unlikely that the interview is serious, when serious people from the side of the company are not participating in it.
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