Basic Solution
You can check what view is activated using router via router.navigationModel()[x].isActive
Application Messaging
You can in the Activation event of ViewModel use Durandal Application-Wide Messaging
In view model in Activate function include
app.trigger('viewNavigated', isBackVisable);
While in shell view model
app.on('viewNavigated').then(function(isBackVisable)
{
if(isBackVisable)
MakeButtonBackVisable();
});