Short answer
The reliable way is the URL Inspection tool in Google Search Console, which says whether a specific page is indexed and, if not, why. Asite:yourdomain.com search gives a rough picture, but Google says it “doesn't necessarily return all the URLs that are indexed.” The quick check: a site: search
Type site:yourdomain.com into Google. You will see pages Google has indexed from your site. It is a fair first look, with two limits Google spells out: the operator “doesn't necessarily return all the URLs that are indexed”, and “bigger sites shouldn't expect to see all their URLs in the results” ( Google). So a page missing from the list isn't proof it is missing from Google.
The reliable check: Search Console
- Add your site to Google Search Console and verify that you own it. It is free.
- Paste a page address into the URL Inspection bar at the top.
- Read the result. It tells you whether the page is on Google, and if it isn't, the reason: blocked, marked noindex, a duplicate of another page, or not crawled yet.
For the whole site, the Page indexing report lists which pages are indexed and groups the rest by reason.
If a page isn't indexed
Fix the reason first, then use Request indexing in URL Inspection. Google adds that “there's a quota for submitting individual URLs and requesting a recrawl multiple times for the same URL won't get it crawled any faster” ( Google Search Central), so ask once and give it time.
Being indexed is necessary for Google's AI Overviews too: Google says a page must be “indexed and eligible to be shown in Google Search with a snippet” to be linked from one.