When we talk about how your website looks on mobile devices, what sort of device do we mean? According to Statcounter, the most used mobile screen sizes are Android’s 360×640 pixels and Apple’s 375×667 pixels, accounting for over 50% between them. These are smaller sizes than you might think are typical. You can see in Google Analytics what users to your site are using (Audience > Mobile > Devices > Screen Resolution). If you don’t have a phone of that size to hand, you can simulate what happens in your desktop browser (in Google Chrome, View > Developer > Tools, select the device icon). There are of course much bigger mobile device resolutions, and it’s worth checking how things look in these.
What about Google though? How does the search engine’s crawler see your mobile site? You can check that in Google Search Console (Crawl > Fetch as Google > Crawl and Render). The results also show if Google reckons it’s seeing something different to normal visitors. It’s worth checking out.