This one is quick and easy. You need to know the record info that was setup with the DNS Provider to do it
Go to https://MXToolBox.com/NetworkTools.aspx
Find the DKIM Box
Enter in the domain:selector info
Example – tacticalware.com:mydkim
Press the DKIM Lookup button
If it is setup properly you will see in green, the dkim information. If it is setup improperly, you will see an error, as you would see if you searched for the dkim tacticalware.com:mydkim