DKIM checker
Enter a domain and, if you know it, the selector. Without one, the common selectors of Google, Microsoft, Proton, Zoho, Mailchimp and others are tried.
Finding the selector
Every DKIM signature names its selector in the s= tag of the DKIM-Signature header. Send yourself a message, open its headers and look for s=, or paste them into the header analyzer.
What DKIM does
DKIM signs every outgoing message with a private key. The matching public key sits in DNS at selector._domainkey.your-domain, so receivers can check that the message was not changed and really comes from your domain.
Use RSA keys with 2048 bits, publish one selector per sending service and remove old selectors only after no mail is signed with them any more.