how can i check this (is it working or not)

Hi,

You can check (before + after), for example on:
https://developers.google.com/speed/pagespeed/insights/
https://gtmetrix.com/

You need to be logged out from wordpress, to see the FVM generated files.
The option to fix page editors, in the settings, will disable FVM for logged in users for better compatibility with page editors, such as the visual composer and others.