What to run
Use Is This Website Down, HTTP Status Codes, and HTTP Headers. The down check and the headers are from Cloudflare. HTTP Status Codes stays in the browser.
How to read the result
200 with a tiny body is still up.
401/403 is up and refused.
A redirect to an error host is up.
Headers may show a challenge cookie or a maintenance header.
What to open next
If the final URL is not the one users wanted, open Redirect Checker. That result is from Cloudflare.