When you are offline, you will see a notification in the top left of your screen.
During a sync, you will see the rotating symbol in the Refresh button animated. Further details during a sync can be found in the Operations window, which you will find by clicking on the down arrow in the Refresh button.
I found it somehow reassuring to see after pressing refresh: to confirm that something was actually happening. At least if someone sends a huge file, the arrows turn as it downloads, but it would be good if it did it on every refresh.