If you wish to see debugging information about a certain page, perhaps trying to find out why an asset isn’t being optimised by Pagespeed, you can view source on the page you are trying to debug.
?PageSpeedFilters=+debugto the view-source URL.
You should now see comments next to an assets that indicate reasons why is was not optimised. e.g.
<!--Resource headers are preventing rewriting of https://www.example.com/123.gif-->