ZoomService allows you to register a listener for receiving Zoom change events. Every time when zoom level for a specified web page has been changed, the ZoomChangedEvent will be fired:

C#

// Listen to zoom changed events
browser.Context.ZoomService.ZoomChangedEvent += delegate(object sender, ZoomEventArgs e)
{
  Console.Out.WriteLine("e.Url = " + e.Url);
  Console.Out.WriteLine("e.ZoomLevel = " + e.ZoomLevel);
};

VB.NET

' Listen to zoom changed events
AddHandler browser.Context.ZoomService.ZoomChangedEvent,
    sub(sender As Object, e As ZoomEventArgs) 
        Console.Out.WriteLine("e.Url = " + e.Url)
        Console.Out.WriteLine("e.ZoomLevel = " + e.ZoomLevel.ToString())
    End sub