[sparkling]

$ dig example.com

to get the A Record, then

$ whois 1.2.3.4|grep Cloudflare

Not 100% reliable, but should do the Job.

[neuland]

    whois $(dig +short yoursitehere.com) | grep -i 'Cloudflare' 1>/dev/null; if [[ $? -eq 1 ]]; then echo 'Didnt find CloudFlare'; else echo 'Found CloudFlare'; fi

Not at a terminal now, but this long one-liner should work.

Like you said, not 100% reliable though. For example, I'm pretty sure Reddit uses CloudFlare, but their whois mentions Fastly, which is a competitor.

[gooeyblob]

We moved off of CloudFlare to Fastly before this vulnerability.

[nikisweeting]

Confirmed, reddit.com was removed from the list. My mistake for not double checking this one.

[nikisweeting]

If you find any domains with this please add them to the list:

https://github.com/pirate/sites-using-cloudflare