Participants have the option to join your call as just a viewer, not hearing or being heard. Organizers can see who these viewers are by their participant card not having a Microphone icon to be muted.
Also, once you’ve joined your conference, click on the Participants icon on the left-hand sidebar to display your participant's list. Those who are simply viewing will also not have the microphone icon here.
Need to evict a viewer? As the Organizer, click on the X icon in their participant card to remove them from your call.