From a developer's point of view:
HTML5 does not consist of semantic tags, but semantic tags are the new way to go in developing HTML5. Still HTML5 is an upgraded form of HTML4. It differs in many ways or in less ways then expected. There are some differences that you look for like (again) semantic header, section, footer tags or self closing tags like which in html5 can be (although it validates both ways).
Main point is if it has <!doctype html>
or the xhtml version <?xml version="1.0" encoding="UTF-8"?>
at the top of the file and if it validates I'dd consider it html5.
You can also read all the differences on w3.org which might give you a better understanding of what to expect.