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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.
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.