How do websites know they're not the default home page or search provider?

As far as I'm aware, there is no public API exposure of a browser's default homepage/search provider. So how does Google know to display this? It only comes around when Google's not the default homepage / default search provider on my browser.

I can only assume they're inferring from numerous variables, such as the referrer. I wasn't able to successfully dig down into Google's compiled JavaScript. I'm not even sure if it's detected client-side or server-side.

I'm on Firefox 44, but I've seen these banners on Chrome, too.

Simply there is no way to do that with JavaScript because the "default search/homepage" is a user's preference and you do not have access to that without user's permission because that would be a security/privacy issue.

What Google does at every user visit is show a promo ad with a close icon and a go button with instructions on how to set it as the default homepage. On click of any one of them, it creates 2 cookies so that next time it will check your cookies and make the promos disappear. Even when Google is your homepage and you clear your cookies then a banner is still there to promote Google as your homepage.

I have checked this with Firefox, not aware of Chrome.

I don't know what Google does, exactly, but what I would do:

• set the homepage URL with some special parameter and check it - 'http://www.example.com/#!homepage (prevents false negatives)
• check for Referer field:
  • if it's NOT there, assume user has typed it in manually
  • if it's very similar for each user visit (and perhaps at what looks like the beginning of a browsing session - inferred via GA on eeevery page out there), assume user is coming here by always clicking through from somewhere
• set a cookie, e.g. visitedHelpAboutHomePage when the user visits the "yes, show me" page (might prevent false negatives, but might also generate false positives)

Note that the "special parameter" does happen in the "searchbox-initiated search" scenario: there is a parameter sourceid which likely means "source of search."

In my Firefox (v42), the "Come here often?" box appears even when I have Google set as the default search provider.

In Safari I get a box in the same location that says "A better way to browse the web: Get Google Chrome"

In Opera and Chrome I get no box at all, even if I change my default search provider to something other than Google.

So, I believe all they're doing is browser detection in the JavaScript, using the UserAgent object.