There is a process to messaging and I think many folks are overly focused on the last step which is the final translation to words. I actually think this piece is the simplest of all the steps so I don't spend a lot of time looking at documented work when I interview product marketers. What's really important to me is understanding how a candidate thinks about and navigates the process itself. How does someone internalize the needs of the customer and market dynamics. In many ways, this is more about understanding how someone synthesizes data (both qualitative and quantitative) and builds empathy for their customer. I also look for candidates that can tell a cohesive narrative about their own career choices to date. After all, if you can't tell me a compelling message about yourself, you're going to struggle when it comes to B2B.
For success metrics, I primarily look at usage in the field. How many downloads and views is your messaging getting? If you have landing pages or blogs with the message, how do those metrics compare to others on your website? Lastly, I take a lot of qualitative feedback from customers and our field on messaging. This is more difficult to show leadership but is the best internal measure for if it's working or not.