My friend Ronald has a excellent post where he uses XML to gather entities from the various extensions to check if they are installed. Awesome stuff! The problem though is that it requires Javascript to be successful. I wanted a way to check any extension even if they had Javascript disabled or noscript installed. The following POC detects noscript even when you have a site as untrusted.
Check the POC here (detects just noscript at the moment):-
Total Recall noscript
kuza55 (Alex) also did a great POC with CSS:-
CSS extension detect