There are dozens of legit ways to automatically detect this, and dozens more if humans are involved.

Requesting non-mobile versions of sites that do not have the option

User Agent strings such as "Internet Explorer" or "Safari" in HTTP requests

Sending screen sizes via relatively common web calls

The use of UA-Pixels at all, especially when specifying large screen sizes.

Use of protocols that are only seen in desktop OS programs (ventrilo, starcraft2, etc for instance is one that should be a good detector).