DKIM checker
Looks for DKIM keys under the selectors the large providers use, or the one you give it, and reports each key's size and whether it has been revoked.
The rest of the domain
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What a selector is
A DKIM key lives at selector._domainkey.yourdomain.com. The selector is chosen by whoever sends your mail, and DNS gives no way to list them, so a checker can only try known names.
To find yours, open a message you sent, view its source, and read the s= value in the DKIM-Signature header.
RSA keys under 1024 bits are rejected by receivers. 1024 still works but 2048 is the current recommendation.
Other checks
SPF checker DMARC checker DKIM checker MX lookup Full domain report