Watch position and impressions, not just the traffic line, because decay shows up there first. By the time organic sessions visibly fall, the slide has usually been underway for weeks. The headline traffic number is a lagging indicator. The leading indicators sit upstream of it, in the metrics that move before a single click is lost, and a page is genuinely worth monitoring only if you are reading those.
The earliest signal is average position. A page drifting from the top of page one toward the bottom often holds its traffic for a while, because the top results still absorb most clicks, but the drift is the warning. Impressions are the next tell. When impressions for a page start declining, your content is surfacing for fewer queries or sliding down on the ones it still appears for, which precedes the click loss that traffic finally registers. Watch click-through rate too, since a falling CTR at a stable position often means a competitor’s richer title or snippet is winning the click before yours.
Look outward as well as inward. Rising competitor freshness is a leading signal you will not see in your own analytics: when newer, more complete pages enter the SERP, your position erodes before your traffic does. Secondary and long-tail terms decay first as well. A page usually loses its supporting keywords before its primary one, so a quiet drop in the number of ranking queries, visible in Search Console, is an early tremor before the main term moves.
Set up the habit now. In Search Console, track average position, impressions, CTR, and the count of ranking queries per page over rolling windows, and flag any page where position is drifting or impressions are sliding even while clicks look steady. Pair that with a periodic glance at who has newly entered your top SERPs. Treat those movements as the alarm and revisit the page then, rather than waiting for the traffic graph to confirm what the leading indicators already told you.