tl;dr -- define what you need this person to do, then ask for specifics of opportunity identification and execution during the interview.
I think this really depends on the level of seniority/breadth of responsibility you're hiring for, so the first thing I'd recommend is getting crystal clear on what you actually need this person to do. Then I'd make sure that you are aligned with the recruiter on the req (if you have a recruiter) and the rest of the hiring panel all agree on what responsibilities and success look like for this role.
This is important because responsibility-relevant autonomy takes many forms. For example, if you're a 50-person startup making your first PMM hire you need that person to be able to develop a functional vision, establish an internal cross-functional operating model, and prioritize what to execute against given constrained resources and rapidly shifting needs of the business as it grows. This is very different from, say, what you'd need if you're hiring a fresh MBA to fill a role on your team at Microsoft -- especially so since at a larger company this person might be more responsible for coordinating external contractors than on the line execution.
Once you and your team are aligned, the best way to gauge autonomy is to ask someone to walk you through an impactful project they're proud of that reflects their work. Make sure to use follow-ups to:
The more specifics you can get to the better! Further, it's usually a sign of autonomy and leadership that the person will naturally be forthcoming with some if not all of the above bullets without too many follow-up questions. If getting to answers for the above is like pulling teeth, this is probably not the right candidate for you.