There are a few ways you can check whether someone is in the Room with you:
1. You will see a grey notification box pop up in the top left hand corner of your screen notifying you when someone is already in or enters the Room.
2. You should be able to hear them. If not, it may because either they or you are in Silent Mode or are having voice connection problems.
3. You can always check who is in a Room with you by clicking on the ? icon in the toolbar. The names of present participants will appear on the information panel.