Are you looking for an answer to the question: “How do I try a mock request from ALL Cloudflare’s datacenters (all 165 of them…) to my specific site, to see if it’s successful?” I personally did not see an option like that anywhere (that doesn’t say one doesn’t exist…)
Generally “the Internet” is responsible to get your traffic to your site. You wouldn’t ask the same question for any of the other billions of Internet-connected hosts, right? Even though they’re likely much more poorly connected (likely Cloudflare peers with Tier-1s and more)
If however you’re asking if a particular data center now has an issue reaching you, how you’ll know? Probably your users will have to complain somehow: and Cloudflare will tell them that the issue is between Cloudflare and your server - a diagram will be shown that looks like this:
Your Browser -> Cloudflare (Data Center Name) -> Your Host
(working) ----------- (working) ------------ (unreachable)
And if they get nothing at all, that means they can’t get to Cloudflare’s data center.