This is often a good idea regardless. Several years ago, I was in talks with a YC startup to join as a technical cofounder early in the program. I reached out to about 4 other YC founders or early employees who had gone through similar situations - not being a member of the founding team, but joining as a cofounder or employee #1 while the startup was still going through YC. Eventually decided to pass, which turned out to be a good idea in hindsight since the startup imploded in a founder breakup soon after the program ended.

The people I reached out to didn't even know or speak directly about the founders or startup itself. But because they had been in similar situations before, they knew what to look for that might become issues in the future, and then I could cross-reference that with my own personal experience to make a judgment.