Q: How do I know if God is calling me to be a practitioner?
A: God is calling you—God is calling everyone, including every Christian Scientist—to be a healer. In fact, it's a mistake to think that there are a few privileged Christian Scientists who are called to be healers, and others—the majority—who aren't. The model of church that Mary Baker Eddy created didn't include two types of members, but just one.
We wouldn't say "How do I know if God is calling me to be a child of God?" Why? Because we're all children of God, not just some of us. God calls us all to be who we truly are.