Cloudflare Checker
Just input a URL, and we'll tell you if that site uses Cloudflare (which is known to block bots).
Est run time: 15-20s.
Usefulness: Cloudflare is known to block bots, so this can be a good way to detect if a site uses Cloudflare before you build a complicated scraper against that site.
Caveats: This is generally a good way to detect if a site uses Cloudflare, but it's not a definitive test.
Additional: We made this mainly for ourselves to detect Cloudflare users, but perhaps someone out there has a thing for knowing who uses Cloudflare or not.