Ask HN: How to identify a website built with Bootstrap?

Markup is the way to go, look at classes of buttons, divs and spans. Look for -lg or spans and stuff like that.

That... and you could open up the dev console

Checkout http://builtwith.com. If the css is not renamed, merged or minified it will be listed in the technologies used. Else, you have to do the one of the other suggestions here.

View source and look for a CSS file named something like "bootstrap".

Usually this is the best way.If the bootstrap.css has been renamed you can download bootstrap.css (or minified version) file,open css file in the website source, then [Ctrl+F]+[Ctrl+V].

I'd also like to suggest a really cool browser plugin called Wappalyzer. Its really good (ok, kind of decent) at guessing the platforum of a website and will even guess plugins, frameworks, CMS, web server and I think sometimes even host.

A mildly decent way is to check for `span[0-9]+` or the newer `col-` whatever classes. A site using classes of `container` and the ones previously mentioned probably has a 80%+ chance of using the bootstrap grid.