How to assess the proportion of post docs in a lab that go on to become PIs?
One approach is to simply ask the potential post doc supervisors. That said, if I received a random email from someone asking how many of my post docs have gone on to get academic positions, I would probably ignore it. If however, that email came from someone I had previously talked to about doing a post doc with me, I would be happy to tell them about my past post docs and where they went. You should then talk to former members of the lab and see if the PI's view of the world matches their views. While a single descriptive statistic like percentage of students who went on to get academic positions might seem like it is easier to interpret, it likely only provides a portion of the story.