The best product managers can pick a product/feature they led and drill down to the various product angles and accurately articulate:
A. What was the process to define the requirements
B. What was the process to productize the requirements
By zooming in to the fine details and zooming out to the strategy and overall execution you should be able to evaluate a candidate.

When evaluate a PM I usually look for:
- Working in uncertainty
- Clarity of thought
- Data driven/informed vs. Guessing
- Be able to see the entire picture while zooming in to the fine details
- Technical Skills
- Leadership

The list above varies per company as in each industry and in each stage of a company the priorities are different.

Two great posts about product management:
How to hire a product manager by Ken Norton
https://www.kennethnorton.com/essays/productmanager.html
Good Product Manager/Bad Product Manager by Ben Horowitz (appears with more details on his book "The hard thing about hard things")
http://web.stanford.edu/class/e140/e140a/handouts/ProductMgmt.txt