You usually won't know a web page is actually using H5BP, especially without looking at the source.
Two websites using it: http://creativecommons.org/, http://www.barackobama.com/
Two websites using it: http://creativecommons.org/, http://www.barackobama.com/
One of the ways to know, before the new release which sadly kills it, was that many sites didn't customize the default hot pink text-selection background color.
Select text and get hot pink? You knew it was the boilerplate.