How to check if the server is sending content using GZIP compression?

You can use the google extension called pagespeed: link

But in a quick way ... If you use Google Chrome, press F12 and you'll be taken to the DeveloperTools (a small window below your browser). In it you will go to the Network tab and there you will see all the requests made to your website, look for the Accept-Encoding: gzip, deflate key.

If this exists, your site uses GZIP: D

Update:

In Response Headers there is the Content-Encoding key: gzip there should be gzip.