I think from API it will not show whether the user has joined from meeting APP or from endpoint or browser.
But if you have the users list in CMS probably you can check the username from below API-
https://<server ip : port>/api/v1/calls/<call ID>/participants
And can get the details of participants.
In spite of that simple method is login through webadmin and navigate to status--> Calls
And this will show you active conference and the participants (whether its SIP endpoint or CMA).
Please rate if it is helpful....
But, participants are those in a call. How can all CMA users who are signed on be identified -- including those not currently in a call?