Prove It
It’s difficult for knowledge workers to ‘prove’ expertise in a concise and simple manner. We might agree it’s more obvious for a chef, or a cabinet maker, or a tennis player.
A consistent approach to this might help demonstrate ability and competence.
Competency has aspects of knowledge, skills, and experience. If we agree those are a basis for competency then:
Knowledge is assessed by review of training/education; outputs/documents; and an end result.
Skills are assessed by repetitive demonstration; interactions with others; and feedback.
Experience could be assessed through exposure to situations; stories told about those situations; and reviews from others.