Rod Howarth posted a free Add-in that will pop up a dialog and put a flashing icon on your taskbar when your Synchronize with Central finishes. For more details and how to download/install it I'll let you read all about it on his blog.
Keep in mind that if you have Worksharing Monitor installed you can tell it to tell you that it knows that you want to know that your Sync with Central is done too. It's done via the Options here:
I've checked the two settings I prefer but you can check them all so you'll also know when something fails. The notifications do expire on their own so you could miss them. The advantage Rod's app has is that it will require your action to dismiss it. Unless you change the following setting in Worksharing Monitor too.
This is your message to go get messages!