Free tool
The same question to six public resolvers and to the zone's own nameserver. If they disagree, a cached answer is still out there; the TTL column says for how long.
What this checks
When you change a record, your nameserver answers with the new value at once. Every resolver that cached the old value keeps serving it until its copy expires, which takes up to the old TTL. "Propagation" is just that wait. The authoritative row is the truth; the others show who has caught up. Lower the TTL a day before a planned change and the wait shrinks to minutes.
Related tools
Domains and DNS
Register or transfer .lu, .eu, .be, .com and more; the DNS editor, DNSSEC and the records these tools check are in the client area.