Free tool
DKIM selector lookup
Free DKIM DNS lookup: probe common selectors (google, selector1, k1, …) or pass your own — TXT via public DNS.
Optional: append ?selector=google via API. UI checks common selectors automatically.
DKIM publishes a public key in DNS under selector._domainkey.domain. This tool probes common ESP selectors so you can see whether keys are visible before cutover.
How selectors work
Your ESP chooses the selector name (google, selector1, k1, …). The TXT at that name holds the public key. Without the matching private key on the signing MTA, receivers cannot validate DKIM signatures.
If nothing is found
Your provider may use a custom selector. Check their docs and query the API with that selector, or wait for DNS propagation after publishing.
FAQ
Deeper guide: Read: SPF and DMARC explained