mxtoolbox

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