[Andrew]

We wanted to keep syncing unobtrusive, since it happens quite often (one minute after a modification to backup quickly and to make changes available to other clients quickly, and once and hour even if there haven't been local modifications, in case there are changes waiting that were provided by another client).

There is a progress meter in the Sync preferences pane, so if you have a long sync going on, you can check on the progress there. Hopefully syncs are usually quite brief; if you want to know when one is going on, you can do as Sandy suggested and configure the toolbar to show the Sync item, which rotates (slowly, to reduce visual distraction) while a sync is in-progress.