It’s a common functionallity that you always need. I have done this myself before this way:

if(System.Windows.Browser.HtmlPage.IsEnabled == true)

It have worked great in my projects, but today I found out that this isn’t the best approach, as it doesn’t always work. I found out when I read this blog post:

http://blogs.msdn.com/delay/archive/2009/02/26/designerproperties-getisindesignmode-forrealz-how-to-reliably-detect-silverlight-design-mode-in-blend-and-visual-studio.aspx

Go ahead and read it :)