Technical interview is a good place to start gathering intel. Flip it and ask them out about stuff you know. Are they curious, are they knowledgeable. Curiosity is usually a great indicator.

Employers have the same concerns for their potential hires, that's why there are interview processes after all. Leverage these interviews to ask the right questions.

They may get surprised when you turn the table around, just remind them that interviews go both ways.

You can't know that reliably, but the recruitment process typically involves meeting the team you are being considered for, which is at least something. You do however have excellent intel on your current situation, telling you it's time to move on.

Perhaps I should re-phrase it to "most knowledgable person in the room", since there will always be a lot of new things to learn when switching jobs.