Why you find out too late
By the time a failing team lands in your inbox, the project is usually well underway and the damage is done. Teams that are struggling rarely raise their hand early, either because they hope it will resolve itself or because no one wants to be the one who reports a teammate.
The fix is not to wait for problems to surface. It is to ask, briefly and regularly, in a way that makes it safe to be honest.
What to actually measure
A useful health check is short. Ask whether the team is meeting, whether the work is being shared fairly, and whether anyone is stuck. You are not grading the team, you are looking for the early signals that separate a team that is fine from one that needs a nudge.