If new messages are sent or received after the page loads then the tab will show a count of these messages - if you see that keeps going up then that you will know that the page is still sending or receiving them.
You can click on the tab or on the arrow button on the right-hand side to show and hide the list of the messages, and you can click on any of the messages to see the full details.
WebSocket messages can either be text or binary. Once you have selected a text message you will also be able to replay the request with any changes to the contents that you like. This is not currently supported for binary messages.