Check whether a DNS change has propagated by querying multiple public resolvers at once.
“Propagation” is a slightly misleading word. Nothing is pushed out across the internet when you change a DNS record. What actually happens is that every resolver holding the old answer keeps serving it until its cached copy expires.
That expiry is controlled by the record's TTL (time to live), measured in seconds. A record with a TTL of 3600 can be served from cache for up to an hour after you change it. Resolvers that never cached the old value see your new record immediately.
This is why a change looks live for you and broken for someone else: you are talking to different resolvers with different cache states.
This tool queries four independent public resolvers over DNS-over-HTTPS and shows what each one currently returns.
Note that these are public resolvers, not a worldwide survey. Your visitor's ISP resolver may still be caching. Lower the TTL before a planned change if you need a fast cutover.