Is there a DesignMode property in WPF?

Indeed there is:

System.ComponentModel.DesignerProperties.GetIsInDesignMode

Example:

using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;

public class MyUserControl : UserControl
{
    public MyUserControl()
    {
        if (DesignerProperties.GetIsInDesignMode(this))
        {
            // Design-mode specific functionality
        }
    }
}

For any WPF Controls hosted in WinForms, DesignerProperties.GetIsInDesignMode(this) does not work.

So, I created a bug in Microsoft Connect and added a workaround:

public static bool IsInDesignMode()
{
    if ( System.Reflection.Assembly.GetExecutingAssembly().Location.Contains( "VisualStudio" ) )
    {
        return true;
    }
    return false;
}

In some cases I need to know, whether a call to my non-UI class is initiated by the designer (like if I create a DataContext class from XAML). Then the approach from this MSDN article is helpful:

// Check for design mode. 
if ((bool)(DesignerProperties.IsInDesignModeProperty.GetMetadata(typeof(DependencyObject)).DefaultValue)) 
{
    //in Design mode
}