Page-level and site-wide indexability checks
Indexability is the one category where checking a single page and checking a whole site answer genuinely different questions, and where doing only the first is the most common way sites lose traffic without noticing.
Checking one page tells you whether that page can rank
For a single URL the check is a short, closed list: does it return 200, does robots.txt allow crawling, does meta robots or the X-Robots-Tag header say noindex, and does the canonical point at itself or somewhere else. Four questions, four answers, and if all four pass the page is eligible.
This is exactly the check to run when a specific page stopped appearing in search. It is fast, it is definitive, and it usually finds the cause immediately, because the causes of a single page becoming non-indexable are few and mechanical.
Checking a site tells you whether the pattern is spreading
The damaging version of this problem is never one page. A noindex reaches production on a shared template and applies to a whole section. A canonical rule written for one page type gets applied to another and points hundreds of URLs at a single destination. A robots.txt edit intended to block a staging path blocks a live directory that happens to share a prefix.
None of these look different from a correct configuration when you inspect one page. They are only visible as a shape in the data: a directory where every URL is suddenly non-indexable, a canonical target receiving hundreds of inbound canonicals, a status code distribution that changed since the last crawl. Crawlability and indexability are also distinct, and a site-wide view is what separates them: a page blocked in robots.txt was never crawled, so its meta robots directive is irrelevant and possibly never even read.
Compare against what Google actually indexed
A crawl tells you what should be indexable. Search Console tells you what Google has actually indexed. The gap between those two lists is where the real findings sit, and neither source alone reveals it.
Pages that are indexable but not indexed are usually a discovery or quality problem rather than a directive problem: nothing links to them, or Google crawled and chose not to index. Pages that are indexed but not indexable in your crawl are the more urgent case, because something changed recently and the deindexing has not finished propagating yet. Catching that gap early is the difference between fixing a directive and waiting out a recovery.