This is a very subjective question and the answer would very much vary on you personally are looking for in therapy and what fits into your definition of professional behavior.

It's difficult to define the "red flags" because even some of the things that are clearly outlined as unethical in the professional code of ethics such as sexual or sexualized behavior, engaging in business ventures with clients and others of that sort, are perceived by some people as normal. To me, those are not just "red flags", they are ground for legal action. But, astonishingly, quite a few people seem to be unfazed by those things.

The "red flags" that are obvious to me, may not be obvious to others. That's why the simplest way to protect yourself is to give yourself a permission not to tolerate anything that doesn't sit right with you. So many people get harmed simply because they don't trust themselves and their own judgments. There is no need to second guess if you don't like something the therapist is doing. If you don't like it, you don't need to tolerate it. Just leave and look for something you like better.