There are various methods used in verification. For instance, some collectors have their items tagged for identification purposes. This provides a way of tracking changes in ownership of an item. Appraisal estimates the value of an item. Appraisers use many methods including assessment of authenticity and condition.
Appraisers also keep up with the latest on what comparable items have sold for in recent months.
Appraised values are accurate at the time of appraisal but can change over time as items become more or less popular. I know a lot about what I collect already. When large amounts are involved, or when an item for your collection is really important to you, you may wish to consult an expert for confirmation of your own good judgement. Sellers can make their goods more attractive by providing evidence of appraisal from a qualified expert. Serious bidders may wish to negotiate this service with a seller, or may have an item appraised after purchase for purposes of insurance or for other reasons.