A team works well when its goals are clear, people know their roles, they trust each other and they deliver. These questions check each of those in turn. Ask the whole team, including the leader, and compare the answers.
We've ticked a short starter set. Change it to suit you.
Goals
Roles
Communication
Trust
Results
6 questions selected
Tips for a better team effectiveness survey
Discuss the results with the whole team, not just the leader.
Look for questions where answers differ widely. Disagreement is often the real finding.
Pick one area to work on for the next quarter, then ask again.