is_blog_installed()

is_blog_installed() — Determines whether WordPress is already installed. The cache will be checked first. If you have a cache plugin, which saves the cache values, then this will work. If you use the default WordPress cache, and the database goes away, then you might have problems. Checks for the ‘siteurl’ option for whether WordPress is installed. For more information on this and similar theme functions, check out the https://developer.wordpress.org/themes/basics/conditional-tags/ Conditional Tags article in the Theme Developer Handbook.

Returns bool: Whether the site is already installed.