How to assess the proportion of post docs in a lab that go on to become PIs? Is there any good resource to figure out the track record of academic success of research lab’s alumni (e.g. how many of postdocs became a PI)?

I'm a grad student in the US who will finish PhD next spring, and I've just started to find a postdoc position. I realized it's crucial to find an postdoc advisor who has a good track record of alumni's getting job in academia, since I want to be a PI in the future.

It looks like many famous labs with big papers actually have too many postdocs and grad students so that not many of them have successful career. Productivity per person is diluted too much in those labs. I also doubt one's publication record is the only factor for success in getting a PI position as far as I hear from people around me. (Of course, I know that if you have a big paper in a big lab, that is the best way to get a position)

I am open to go to any country for work if the working environment is good. My field is biophysics with some flavor of materials science.
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.