What counts as acceptable evidence of competence?

A training certificate proves attendance. A test score proves recall on the day. Neither proves the person can do the task on site, which is what an auditor, a client and an incident investigation all want to see. The usual answer is a mix: observation of the task, a professional discussion, work products, and witness testimony from a supervisor. In practice observation is the one that gets skipped, because it needs an assessor free at the same time as the work. How do you gather observation evidence without stopping the job, and what do you accept when the task only happens twice a year?