It really depends on how much customization has gone into the site. TechCrunch, Wired, and TIME all use WordPress for example, but their theme is customized to the point where you can't really tell that it's WordPress. There are some ways to tell though, for example some of the larger sites are hosted by Automattic (these say "powered by WordPress VIP" in the footer), and /wp-admin usually still works to go to the login page.