From the perspective of a Product Manager, it's amazing how different PM interviews can be between organizations.
While there are a fair share of rigorous hiring processes, what catches my surprise is how many startups simply do too little to vet a PM. Those who are proud of their work would much prefer a way to display this than an easy offer.
Here's my 2 cents.
Let Them Speak
Most of the intangibles that come with being a PM are often revealed when a candidate speaks to their own accomplishments. When given the steering wheel, which aspects of their products do they speak to?
Having a PM describe their own successes will reveal what they perceive goes into making a successful product:
Speaking to the challenges that were presented and the features they proposed to fix them are indicative of strategic thinkers.
Detailing the struggles of a team or organization first means the candidate is likely more project management oriented.
Getting a sales pitch is common among the pre-entrepreneurial type (does your organization need this, or the skills to execute?).
Receiving prepared materials (such as final delivered products or associated documents) shows confidence in their ability, as well as an understanding that it is difficult to communicate the success of products with simple words.
Push PMs to ask their own questions. As a candidate one of my favorite interviews kicked off with a simple "ask me anything." After 45 minutes of back and forth, the interviewer followed with "well, you clearly know what you're talking about so let's skip the formalities."
Product management is trending towards being a personality-based profession first. This is understandable, considering that PMs will ultimately dictate day-to-day culture on a smaller scale. A single product manager can affect the morale of an entire part of your company.
It's equally important to realize that the technical abilities of a PM mean little if the cultural checkbox has not been checked. An unlikable person cannot effectively lead a team.
Also be wary of overly A-type personalities. While they may be fun to interview with, your team may not have fun working with them.
Stay in Network
Hiring via network is a given, but this is even more the case when it comes to PMs (see the above). Personally, I have taken all but my first job via my network. Interviews set up via recruiters never seem to feel as good of a fit as though organically referred.
Hiring via network gets through the difficult parts of measuring intangibles faster.
Quiz Their Understanding
My favorite PM questions revolve around the moves being made by players in the field. It demonstrates understanding of business and product strategy at the largest scale.
Why would Facebook buy Instagram for 1 billion dollars?
What are the factors behind Snapchat's falling stock prices?
Why did Apple shift to a "flat" design aesthetic?
Tagging friends in Instagram comments now opens a private chat with them. Why do you think Instagram did this?
Give Them Homework
My personal favorite is to let Product Managers put their money where their mouth is.
Give them a scenario, such as building out a feature set for a product. Gauge the effort they expel and see how they choose to present their findings.
This is how you let a candidate walk the walk.
All of the Above
Hiring the right PM is hard, perhaps the hardest of any position in tech. Be sure to put candidates through the paces to avoid undoing the damage of hiring the wrong person.