How to check whether this plugin is in use or not?

Hi,
Check if there are any entries under Settings → Header and Footer scripts.
See https://www.digitalliberation.org/docs/header-and-footer-scripts/ for details.
If you’ve got CSS, then if you’ve got a modern version of WordPress, you can now use Appearance → Customize → Extra CSS instead.
If you’ve just got the script for Google Analytics, then there are alternative ways to insert it’s script. (Although using this plugin is also OK, in my opinion, although I’m not related to it, I’ve just been using it on a couple of sites.)