Why ads.txt is the useful signal
Publishers who sell ads programmatically put a file at /ads.txt listing every company allowed to sell their inventory. The IAB introduced it so buyers could tell real inventory from spoofed, which means it exists for the site's own benefit and is kept current. Reading it is not a trick: it is a public declaration, and a far better source than counting ad slots on a screenshot.
We pair it with a second, independent check — whether the homepage actually references ad network code. A site that both authorises Google in ads.txt and loads the AdSense script is running ads. A site with one but not the other is the interesting case, and the result tells you which half is missing instead of averaging them into a number.
What this can't tell you
We fetch the page as served and do not run JavaScript. Sites that inject ads after load — through a tag manager, a consent gate, or a lazy-loading wrapper — can be earning well while showing us nothing. Read a negative result as “no ad code in the HTML we received”, which is exactly how the result is worded.
Display advertising is also only one way a site makes money. No ads.txt says nothing about affiliate links, sponsorships, subscriptions, lead generation or selling a product — several of which pay considerably better. And revenue is a separate question from ads entirely: a site can run ads and earn almost nothing.
Some sites refuse us outright. Bot filtering in front of a site will return a 403 to any automated request, ours included, and then we have read nothing at all. The result says so rather than reporting an absence of ads — a site blocking crawlers is telling you about its security, not its advertising.
Finally, ads.txt is a declaration of who may sell, not proof that anyone currently is. Files go stale. Treat a long authorised list as evidence of intent and setup, not of income.
Checking a channel instead of a site
Websites are the easy case, because ads.txt exists. Platforms publish nothing equivalent, so checking a YouTube channel means inferring from public signals — and checking a Facebook page or a TikTok account means working from published thresholds, because nothing is externally visible at all. The honest answer differs by platform, and we say so on each.