how can you tell if a 500 error is cloudflare's fault or your server's?

if the page says Cloudflare at the bottom with a Ray ID and a plain 500 (not 52x), check their status page. that one is occasionally on their side. note the Ray ID either way, their support will ask for it.

a plain 500 is almost always your server. cloudflare just passes the origin's 500 through, so if the error page looks like your site's (or a bare PHP/nginx page), it's you. when cloudflare itself can't reach or talk to your server you get its own branded page with a 52x code instead: 520, 521, 522, 524 and so on. those still usually point at the origin (down, firewall blocking cloudflare IPs, too slow), but at least you know where the break is. quickest test is to hit the server directly, skipping cloudflare. put the origin IP in your hosts file or pause cloudflare for a minute. if it still 500s, check your server error log.