You can check if they are mobile friendly with this tool-https://search.google.com/test/mobile-friendly
If the theme you are using is not responsive, you might consider switching to a responsive theme. You can read more about responsive design here – https://en.m.wikipedia.org/wiki/Responsive_web_design
If switching to a responsive theme is not possible for some reason, another option is to build a mobile version of the site on a subdomain or subdirectory and redirect mobile users from the desktop site.
1.You need to compress images and minify js and css files.
2.Install a cache plugin.
3.You need to place all js and css in the footer.
4.Then you have to check site with( https://testmysite.thinkwithgoogle.com/) Google.