Begin with the first date on which impressions, clicks or average position moved outside their normal range. Compare that date across countries, devices and page types instead of relying on the sitewide line alone.
Next, separate pages that lost demand from pages that lost ranking. Search Console query and page data can reveal whether fewer people searched, a result moved down, or a different URL began earning the click.
Only then compare the pattern with a documented Google core update window. The useful question is not whether an update happened, but whether the affected search intent and page groups point to a recoverable mismatch.