User agent has nothing to do with how they choose to render the site. Your device sends them other important information that it queries for like features that are only available on desktop browsers. What you could do is attempt to disable JavaScript and see if doing that on a desktop user agent browser will do it.
Unfortunately even user agent is more of a simple way of forcing desktop mode.
One way and easy at that is to test for things like html5 or other features which haven't moved to mobile browsers yet.