I'm afraid there is not a way to view the DKIM status for all domains. Within the interface, you'll need to manage the domain then view its General settings page to determine whether DKIM is enabled. To see whether it's working correctly on messages, you'll need to review the logs.
That said, we do show the domain's DKIM status in the domain's Settings file, which is located by default at C:\SmarterMail\Domains\[domain_name]\settings.json:
You potentially could scan the file system for that setting or use the API
to extract that information.
At this time, the ability to view all domains' DKIM status in the system admin panel would be a feature request. I'll forward the request to the development team and change this thread from a Question to a Proposed Idea to track their response.