(Just a suggestion)
I am not an expert on this but my first thought is that you need to render the website and see if it "responds" to different screen sizes. I would normally use something like phantomjs to do this. 
Apparently, you can do this in python with selenium (more info at https://stackoverflow.com/a/15699761/3727050). A more comprehensive list of technologies that can be used for this task can be found here. Note that these resources seem a bit old/outdated and some solutions fallback to python subprocess calling phantomjs.
The linked google test seems to
- Load the page in a small browser and check:
- The font-size to be readable
 
- The distance between clickable elements to ensure the page is usable 
 
 
I would however do the following:
- Load the page in desktop mode, record each div's style. 
 
- Gradually reduce the size of the screen and see which percentage of these change style
 
- In most cases, from a large screen to a phone size you should be seeing 1-3 distinct layouts which should be identifiable from the percentage of elements changing style
 
The above does not guarantee that the page is "mobile-friendly" (ie usable in a mobile) but it shows if the CSS are responsive.