Set Tools menu item to visible only when a Solution is loaded

Hi there,

I am building a Visual Studio Extension Package with VS 2013.

I have been following the Menus and Commands Sample from the SDK.

I can see that at runtime you can set a menu to be visible or not visible, however how do I go about triggering this only when a Solution has been loaded?

Is there something that I can hook into that lets me know a Solution is loaded? 

Many thanks in advance