Hi glokarwell5,
I don’t think any SharePoint Online cmdlet is available to check the SharePoint Designer (client) login status. For the available SharePoint Online cmdlets, check this link out.
A simple way to check who is logged on SharePoint Designer would be going to the File tab of SharePoint Designer and click Account, find the logged on account under Connected Services. You may find many connected services there, however, only one user account per a SharePoint Online tenant is allowed to be added here.
If the above suggestion doesn’t fulfill your requirement, please let me know.
Thanks,
Evan Zhang