If you have shared a Google Slides presentation with colleagues, then open the presentation, you will be able to see at a glance if anyone else is working on the presentation at the same time. You can chat with collaborators using the pane on the right-hand side of the screen. 

If a colleague that has editing access to the presentation opens the presentation, you will see that another user is viewing the presentation in the top-right corner of the screen. Position the mouse over their image to see their contact card. 

You will be able to see any changes that the other person makes to your slides immediately, and will also be able to see the exact part of the current slide they are working on. 

You can chat with all collaborators currently working on the document.

If you receive a chat message in a presentation while the chat window is closed, a red indicator will show on the chat icon:

Note that the chat is not saved in the presentation – if the presentation is closed, the chat will be lost.