New Blogger or the old version of Blogger as they have to actively choose to switch, but for those who forget, or - more likely - if you're looking at a Blogspot.com blog hosted by Google/Blogger and you're wondering if it's a Blogger Classic or New Blogger blog, a new Blogger Help page provides a hint on how to tell.
Many people will have noticed this already, but you can figure it out from the navbar:
"The old Navbar has "Search This Blog," "Search All Blogs" and "BlogThis!" on the left, and "Get Your Own Blog," "Flag" and "Next Blog" on the right.
For the Navbars on the new version of Blogger, "Search Blog," "Flag Blog" and "Next Blog" are on the left, while signing in and posting options are on the right."