Some people do not know that they are subject matter 'experts'; as they typically have to have respected colleagues state this. My wife and myself are examples - I have told her for years that there is almost no one else at her level, but she dismissed my claims until well known (in their fields) professors started to call her frequently for advice and ideas.

I considered myself competent in some stuff, but never an 'expert'. About 10 years past, peoples in various IEEE societies prodded me to publish stuff and do talks at some of the respective annual symposiums. I still do not consider myself an expert - because I do know some actual experts in some of my professional fields and endeavors, but there are other engineers that do rank me as such, which I have accepted because it gets me more work.

My wife became an expert by solving some difficult problems. She solved these seemingly intractable problems by deep observations, knowing what questions to ask, and knowing whom to ask questions.

I have increased my professional competence by education (both formal and self-taught) and carefully choosing companies and problems to solve. But I am too general of a practitioner to be considered an 'expert'.