from:

http://wpf.2000things.com/2010/12/05/146-use-getvaluesource-method-to-find-the-source-of-a-dependency-property-value/

It’s often helpful to determine the source of the current value of a dependency property. You can use theDependencyPropertyHelper.GetValueSourcemethod to do this.

In the following example,the source for the value of theForegroundproperty alternates between the style and the style trigger,based on the value of theIsEnabledproperty.

<Window.Resources>
    <Style x:Key="redgreenButton" TargetType="{x:Type Button}">
        <Setter Property="Foreground" Value="Green"/>
        <Style.Triggers>
            <Trigger Property="IsEnabled" Value="False">
                <Setter Property="Foreground" Value="Red"/>
            </Trigger>
        </Style.Triggers>
    </Style>
</Window.Resources>
<StackPanel Orientation="Vertical">
    <Button Content="A Button" Height="23" Width="75" Style="{StaticResource redgreenButton}" Name="btnTest"/>
    <Button Content="Enable/Disable" Height="24" Width="100" Name="btnDisable" Click="btnDisable_Click"/>
    <Button Content="Display Source" Height="24" Width="100" Name="btnDisplay" Click="btnDisplay_Click"/>
</StackPanel>

private void btnDisplay_Click(object sender,RoutedEventArgs e)
{
    ValueSource vs = DependencyPropertyHelper.GetValueSource(btnTest as DependencyObject,Button.ForegroundProperty);
    MessageBox.Show(string.Format("Source for Foreground property: {0}",vs.BaseValueSource));
}