You can simply check the source code of your page for the following:
CMSPages/GetResource.ashx?stylesheetname=stylesheet_file = CSS minification is working.
CMSPages/GetCSS.aspx?stylesheetname=stylesheet_file = no minification.
You can also take a look at the response header with and without CSS minification enabled, and focus on the Content-Length, you will see they are different.
The CSS files are not minified physically but programatically on the request so not sure how exactly are you testing Page Speed but when the test is trying to get the physical CSS file, it will never receive the minified version because as I've mentioned before, Kentico is not touching the physical file.