A theme preview should cover several page types. A well-designed homepage can still come with a cramped article layout or a hard-to-use menu.
Open a homepage, a long article, a category archive and a page with a form. Include a page without a featured image and a page with a long title. At a phone width, check that menus open and the layout does not scroll sideways.
Record which layouts come from the theme and which come from plugins or custom templates. Use a staging copy when you need to test settings or integrations. Keep screenshots of the current site for comparison before switching.