DKIM is the one record you can't find without knowing its selector — the label that says which key to use. The DKIM lookup handles that for you. Here's how, using mxhelper.com as the example.

Step 1 — Enter a domain and selector

On the DKIM lookup, enter a domain. If you know your selector (for example default, google, or selector1), type it in the second box. Don't know it? Leave it blank and the tool probes around 20 common selectors used by the major providers.

DKIM lookup form with a domain and the selector 'default' entered, and the Look up DKIM button — Enter a domain and, if you know it, the selector — otherwise leave it blank to probe. (click to enlarge)

Step 2 — Read your DKIM result

For each selector that resolves, the tool shows:

1The selector's status — Active means a valid public key is published (an empty p= would mean the key is revoked).
2The public key itself, plus its type and size (a healthy key is RSA 1024-bit or, better, 2048-bit).

DKIM result showing the default selector marked Active and the published public key — A valid, active DKIM key published under the `default` selector. (click to enlarge)

You don't create DKIM keys by hand — your mail provider generates them and gives you the record to publish. To understand what the signature actually proves and how selectors enable key rotation, read DKIM explained.