You have coached the Team for some time but you are not sure how much you have impacted them, if at all.
You may have an opinion about it, or you may have no clue. Either because you are aiming so high that you cannot see when they make small improvements, or because you cannot link their improvements to your coaching.
In any case, it makes sense to get direct feedback from them.
Explicitly ask for Feedback about your coaching.
Setting up that CoachingContract and reviewing the progress with the sponsor on a regular basis is good. Getting direct feedback from the teams that you coach is better. Teams usually know better than your sponsor what you do well and what you don't; and their feedback usually is more easily actionable.
For that, you can simply use any opportunities you have to talk to the team or to its members privately.
If you want a more formal way, you may organize a short retrospective about the coaching itself. I suggest you get someone else to facilitate that retrospective to avoid any confusion. You may also set up and send an online questionnaire. It takes no more than 10 minutes nowadays.
For example here are some questions I like to ask.
In addition to simply giving you free-of-charge feedback about how you do your job and useful insights into how you could get better at it (some people would take a crazy amount of money from you for that), this practice is extraordinarily powerful because, if done well, it promotes many of the things you are aiming for.
Obviously, you cannot be bothering people asking for feedback all the time, and there sure are some good timings and bad timings for asking for feedback, but unless you really screw this up, it is always a good and powerful thing to do.
Related articles: CoachingContract