If the web server relies on Varnish, then it is particularly important to ensure that your web pages are being cached and that they are served in a timely manner.

To confirm this, you can begin with the following syntax:

# curl -I http://www.example.com/index.html

Having used the -I option to display the headers only, if Varnish is installed, you should see something like this:

HTTP/1.1 200 OK
Date: Fri, 06 Mar 2015 00:59:24 GMT
Server: Apache/2.4.6 (CentOS) PHP/5.5.22
X-Powered-By: PHP/5.5.22
Content-Type: text/html; charset=UTF-8
X-Varnish: 5 3
Age: 16
Via: 1.1 varnish-v4
Content-Length: 97422
Connection: keep-alive

In the preceding example, the most important lines are the following:

X-Varnish: 5 3
Age: 16

Now, let's run through a quick explanation of these values: