The answer - or non-answer as it's not very useful - for 99% of people is "reputation". If researcher X has a track record of good analysis that is supported by the consensus of peer review, that's got to be good enough short of either being an expert yourself and doing peer review, or some sort of formal verification.
Excellent article as always. The audit insights are spot on and hit close to home - when you hear someone passed an audit, my first reaction is "who paid the auditors"? If the party being audited chose and paid for the auditors, you may have a problem there...