How do I check my pages look right on a phone?

The page builder has a Desktop / Tablet / Mobile switch in its top bar. Choose Mobile and the live preview shows the page at phone width, so you see what shoppers see. The template builders under Appearance have the same device icons in their Live preview panel.

You can also tune a section for one device:

1. Click a section to edit it, then pick Mobile in the device tabs at the top of the edit panel.
2. Tick Hide on mobile to hide that section on phones only, or Hide on all to hide it everywhere.
3. Change the background, image, layout, style or counts (such as items per row) for mobile only. Text stays the same on all devices. A field you haven't changed keeps the desktop value; click ↺ to reset it.

Click Save, then use View ↗ to check the real page on your phone.