All communication from the employer is terse and formal
When you are invited to a job interview, it's all about their convenience. They don't give you a choice of two or three time slots and ask you which one works best for you.
The interviews themselves are cold and scripted. No one on the other side of the table wastes their time or energy trying to sell you on the position.
When you go to the employer's facility, the employees don't see to be happy. You don't hear any laughter or friendly banter.
The physical work environment is dark, gray and forbidding. Who wants to work in a place like that?
The hiring manager (the most important person in your relationship with the company) does not inspire you as a leader. They do not seem to care about you at all apart from your ability to warm an empty chair.
The employer stops communicating with you at one or more points in the recruiting process and they don't apologize or explain the delay, nor respond to your follow-up messages.
Even at the end of the process when they've told your they want to hire you, you feel no warmth or good humor from the people you're dealing with. To them, you are just another box on the organizational chart.
You can imagine yourself surviving the job, but not thriving or having fun in it.
Your gut recoils or shrivels up at the thought of working there. Always trust your gut instinct!