The market share for Odoo increased by 0.005 percentage points (from 0.031% to 0.036%) for all sites, more than any other segment for the same time period.
A CMS will sometimes include a meta tag within the html of each page. This is usually a "generator" tag like <meta name="generator" content="What CMS" /> but could also be "application-name", "Powered-By" or anyting else the CMS creators decide to use.
Headers
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.