How do I know if I'm a part of family sharing?

If you open the Settings app on your iPhone and there's a "Family" option under your Apple ID (name) at the top, rather than a "Setup Family Sharing" option, then you are part of Family Sharing group. You can tap on the "Family" option to see who else is part of the Family Group, and what is being shared if anything and leave the family sharing group if necessary.

See information in the following (you can do that to see if you are in Family Sharing without taking the the final step of "Stop using Family Sharing"). Leave Family Sharing - Apple Support