A Google experiment shows a special icon next to smartphone-optimized search results. Obviously, the icon is only displayed when searching from smartphones.
Back in 2012, Google ran a similar experiment, but it never became a regular feature.
Google has a few articles about building smartphone-optimized sites. Google supports three different configurations: responsive design (same code, same URL - Google's recommended configuration), dynamic serving (different code, same URL) and separate mobile sites (different code, different URL).
{ Thanks, Herin Maru. }