The following command will display all header information for a request to the site:
cURL
curl -sI https://yourwebsite.domain
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:
HTTP headers without CloudFlare
HTTP/1.1 200 OK
Date: Mon, 17 Apr 2017 07:03:58 GMT
Expires: Mon, 17 Apr 2017 07:13:58 GMT
Cache-Control: public, max-age=600
ETag: "OVVDww"
X-Cloud-Trace-Context: 4bc589fab4a319f41fb7aa2eeb7fcf81
Content-Type: text/html
Server: Google Frontend
Content-Length: 8714
Now once the site (or rather the DNS) switches over to CloudFlare the headers change to something like this...
HTTP headers with CloudFlare
HTTP/1.1 200 OK
Date: Mon, 17 Apr 2017 07:04:14 GMT
Content-Type: text/html
Connection: keep-alive
Set-Cookie: __cfduid=d4161c4f0a3ddd95b561a8921dccecebf1492412654; expires=Tue, 17-Apr-18 07:04:14 GMT; path=/; domain=.yourwebsite.domain; HttpOnly
Expires: Mon, 17 Apr 2017 07:14:14 GMT
Cache-Control: public, max-age=600
X-Cloud-Trace-Context: d1a32aaff5d361cdfec9fd1e9cacaf57
Server: cloudflare-nginx
CF-RAY: 350d87709c3632a1-HKG
Specifically these are the new headers specific to CloudFlare...
HTTP headers with CloudFlare
Set-Cookie: __cfduid=d4161c4f0a3ddd95b561a8921dccecebf1492412654; expires=Tue, 17-Apr-18 07:04:14 GMT; path=/; domain=.yourwebsite.domain; HttpOnly
Server: cloudflare-nginx
CF-RAY: 350d87709c3632a1-HKG
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:
cURL
curl -sI https://yourwebsite.domain | grep Server
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.
HTTP headers with CloudFlare
Server: cloudflare-nginx
-i
