For each page on my site with a mobile equivalent, I use the following HTML tag on the desktop version:
<link rel="alternate" href="(mobile)" media="only screen and (max-width:500px)">
and I use the following HTML tag on the mobile version of the same page:
<link rel="alternate" href="(desktop)" media="only screen and (min-width:720px)"/>
In my real pages, I replace (mobile) and (desktop) with the URLs to the mobile version and desktop version of the same page. From my observations, google appreciates this.
I use an 800x600 laptop screen resolution when working on my site, and when I searched for my site with the query "site:example.com" in google, it shows me both the desktop AND mobile version of the homepage.
Since I specified media values, I would think that google would only show the desktop version of my page since my screen resolution is greater than 720 pixels.
I ask because my mobile site for a desktop user would look a bit funny and my desktop site for a mobile user would look awkward since they would have to scroll all over to read paragraphs of text.