What defines a successful Scrum Master is how much they can help the organization increase the speed of feedback. As Antti says: “we often don’t really know what the customer wants!” So, measure your success by measuring the speed of feedback. Is it increasing?
Antti is an agile practitioner, who got started with agile in my own very first Agile project way back in 2004. He's been through all kinds of roles, from team member, to management, to customer-facing roles.