I would say: - Look at what they have accomplished. But be sure you are looking at accomplishments of the actual team, not people who are taking the credit (that is a little trickier). - See if there is relaxed communication and trust. - Ask everyone individually what the goal of the team or the goal of the project is. Really talented teams sometimes put their individual goals ahead of the overall goal. As in: the front end engineer says a responsive front end is most important, an AI dev. says deep learning is the key feature etc. All can be talented and well meaning. But if everybody doesn't agree on the trade offs, effectiveness will suffer.