Before you install a responsive WordPress theme, you will want to see how it looks on mobile devices. The easiest way to do this is by browsing the theme demo using your mobile phone.
Some WordPress theme shops may already have theme demos with a toolbar allowing you to switch to mobile view directly from your desktop browser.
You can also use the Inspect tool in your browser to see how the theme demo looks in different screen resolutions.
There are also online cross-browser testing tools that you can use. Simply enter the theme's demo URL into any of these tools to run the test.