Version histories of procedure documents naming the employee, records of revisions made after incidents, decision notes explaining changes, and escalation logs showing which questions reach them. Training attendance records help set context, but on their own they describe a teaching workload rather than advanced process knowledge.
Show the process changing over time
A procedure revised repeatedly, with the reasons recorded and the same person involved each time, tells a clearer story than a polished current version. Ask the process owner to pull the change history and pick three revisions that show real judgement. Where an organization keeps poor records, a dated written account from the manager who approved the changes can stand in, provided it is specific about what was decided.
Add one source of evidence that process owners rarely think to offer: the record of what went wrong. Incident reports, post-project reviews, deviation investigations and the notes explaining why a method was changed all show judgement being exercised under pressure, and they are dated, made for another purpose, and difficult to characterise as self-serving. A procedure's revision history explains what changed; the failure that prompted it explains why the person's knowledge mattered.
Hypothetical example: a precast erection methods specialist's file includes three post-incident reviews spanning six years, each recording a method change the same person proposed, which together demonstrate depth more convincingly than the current version of the procedure could on its own.