Use at least three checks: compare the row count with the displayed result count when one exists, deduplicate canonical product URLs or identifiers, and inspect samples near the first, middle, and final part of the collection. If the source shows 300 results and the export has 24, the job is not finished.
For a recurring collection, track count changes over time. A sudden drop may be a genuine assortment change, but it can also be a new filter, layout, or loading behavior. Flag it for review rather than silently accepting a smaller dataset.