How do I tell if a site is using CloudFlare reverse proxy services?

Method 1 (Most Accurate)

Visit the IP you receive upon DNS lookup:

$ ping authy.com
PING authy.com (104.16.0.17) 56(84) bytes of data.

So, going to 104.16.0.17 in my browser gives me this: Direct IP access not allowed

If you would want to automate this, I'm sure you could use curl or a similar tool, but on an individual site-by-site basis, this is the easiest way I can think of doing this.

NOTE: Someone could "spoof" the page into looking like a Cloudflare page, when it isn't, so this method isn't foolproof or guaranteed to work.

Method 2 (Less Accurate)

Take a look at the whois data for any website suspected of using Cloudflare:

$ whois authy.net|grep "Name Server"
Name Server: LEE.NS.CLOUDFLARE.COM
Name Server: MARY.NS.CLOUDFLARE.COM

Anything that ends in NS.CLOUDFLARE.COM is Cloudflare, however, the site owner may not be using the reverse-proxy server (aka the server is gray-clouded), so they are just using Cloudflare's DNS servers.

Another way Method 2 may not work is if the server's DNS is CNAME, which it will look like a private/non-Cloudflare DNS server, but when queried, it will return Cloudflare.

Conclusion: You shoud use Methods 1 and 2 to be fairly certain they are using Cloudflare's reverse-proxy server.