You can view your site from your desktop computer as it would appear on a mobile phone. Simply click the icon that looks like a computer to the right of the settings tab.
The menu is now collapsed on the left side. Navigate to each page and make sure the site looks as you intended. This is especially helpful to check when columns are used. In the mobile view, all columns appear in a straight vertical line.
On the right side are options to change the color of the header area, include the site title (not available on all themes), and toggle on and off the header images, search box, social icons, and phone number. Turning off the header image is often a good idea as mobile users typically prefer content over non-essential graphics.
Once you are finished with the mobile view, click back on the same icon at the top of the screen and you will return to the desktop view.