What happens when an extension is updated on EW?

What happens when an existing extension is updated to a new version on Extension Warehouse? Some alert is send to the user that have that extension installed? Something else like this?
There is any tool that guarantee that the user have the last version of each installed extension?
Thanks a lot guys.

If you launch the Extension Warehouse from within SketchUp and navigate to your installed extensions, you will see red notifications for outdated extensions.

If you use the SketchUcation plugin store to install some of your plugins, you will see the updates for all (with this tool) installed plugins too.

The EW toolbar icon will have a orange circle with black refresh arrows overlay-ed upon the normal button image.
But you have to actually open up EW, as Cotty says above, to see which extension needs updating.