The market share for Medium decreased by 0.01 percentage points (from 0.014% to 0.004%) for all sites, more than any other segment during the same period of time.
HTTP headers can provide some of the best clues about a site's CMS. The values of set-cookie and x-powered-by headers are the most common and easiest to find.
Markup
If a CMS doesn't have meta tags, it will often have other uniquely identifiable html markup. This can range from unique html comments, ids or classes.
Javascript
Similar to identifiable html markup, a CMS will sometimes insert unique Javascript code into web pages.
Domain
Some content management systems also offer hosting. A website hosted on a CMS developer's domain is a dead giveaway.