Find DKIM key

DMARC data sent to your account at dmarcian should eventually show whether your traffic has DKIM signing and what the public key is.

However to check right away, you should be able to: send yourself an email (maybe to a second email account hosted elsewhere), and check for the DKIM-Signature header(s).  If the email is signed, that message will have information indicating what DKIM signature is being applied to your message.  If the domain indicated in the DKIM-Signature header does not match the domain in the From header, that DKIM signature cannot pass DMARC.  You'll need to get that updated, or add a relevant signature (there can be more than one). 

Likewise, if there is no DKIM-Signature at all, there is work to be done if you want these messages to pass DMARC via use of DKIM.

Please review available DKIM articles for further guidance.