To the right of the person's name in the user list you'll see an icon for Camera . If it is grey they are using a cam .  However if you see a locked Padlock then you will have to ask their permission to view their broadcast.
To view someone's camera, click on the camera icon if it is locked a request will be sent to the person letting them know you wish to view it. They have the option of allowing or denying your request.
Please note - it is thought impolite to ask to view another member's camera if you don't have yours on.