The following command will display all header information for a request to the site:
The output looks similar to this when your site is not on CloudFlare, of course this will vary from server to server, the main point is there aren't any CloudFlare specific headers present:
Now once the site (or rather the DNS) switches over to CloudFlare the headers change to something like this...
Specifically these are the new headers specific to CloudFlare...
Out of all those new headers we only really care about the value for the Server header, so the following command is what we're really after:
Running that produces the following output and you can clearly see that the CloudFlare nginx server is being used to deliver your site's content.