Some of the inputs to consider when answering these questions are a combination of subjective and objective measures such as manager assessment, peer reviews, leadership perception, quantitative impact, etc.
When talking with your PMs, try to give them concrete examples of where the gaps are, what you expect from them, the evidence you need to see to know they are performing at a certain level, and examples and resources that will allow them to get to the expected level. Always use your judgment based on your company context.