How do I know what cache file is being used for displaying a page?

If you want to check what cache file is being used for displaying a page, then you should edit your wp-config.php file, change the WP_DEBUG setting and the WPO_CACHE_FILENAME_DEBUG to true

define(‘WP_DEBUG’, true);
define(‘WPO_CACHE_FILENAME_DEBUG’, true);

Then, purge the cache, reload the page and view the HTML source. Scroll down to the bottom, and you should see a line added by the cache:

<!-- Cached by WP-Optimize (gzip) -https://getwpo.com- Last modified: Wed, 01 Dec 2021 10:47:38 GMT --><!-- WP Optimize page cache debug information -->
<!-- 
Extension: .html
Filename: index.html
 --->