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.
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.
Some content management systems also offer hosting. A website hosted on a CMS developer's domain is a dead giveaway.
Dedicated Content Management System Detection