Recently, when I opened a Xamarin project in Visual Studio, I received a message that an update for Xamarin for Visual Studio was available. The Xamarin icon appears in my SysTray, but clicking on it does nothing. Really?
The Xamarin for Visual Studio extension appears in Extensions and Updates, but you can't update it from there either. The solution to get the update is to go to Tools | Options | Xamarin | Other. In this dialog, there is a Check Now link. Clicking on it will bring up a list of updates that are available.
Some people report finding the Check Now link under Android Settings and iOS Settings. It may depend on the version of Visual Studio, or on the version of Xamarin that is installed.