I agree thinking out loud is a way to get this information but it can put candidates at disadvantage. Also, it is not the only way.
As for prompts/clues, the interviewer has control over what they ask and how they ask. Further, if clues are a concern, then they could keep a tally of the prompts and score the candidate up/down based on the number of prompts.
To me, the interview is about understanding why the candidate is doing/proposing what she is doing/proposing. So, while they may not arrive at your expected solution, I am interested in knowing if they employ good tactics/approaches/methods to understand the problem/solution space and arrive at/improve a solution.
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