WPF: How add check mark to the textblock?

You can use the SQUARE ROOT character in either in XAML or code-behind to achieve a check mark symbol:

XAML:

<StackPanel>
    <TextBlock Text="√"/>
    <TextBox Text="√"/>
    <Label Content="√"/>
</StackPanel>

Code-behind:

txtBoxName.Text = "\x221A";

just refer this document by Josh smith about Differences between Label and TextBlock

http://joshsmithonwpf.wordpress.com/2007/07/04/differences-between-label-and-textblock/

I think you can do by using Label

<Label>   
<StackPanel Orientation="Horizontal"> 
<Path 
                                Width="11" Height="11" 

                                SnapsToDevicePixels="False" 
                                Stroke="red"
                                StrokeThickness="2"
                                Data="M 2,4 C 2,4 3,5 5,13 C 5,13 5,3 12,0" />
                                <TextBlock Margin="5,0,0,0">Successfully Completed!</TextBlock>
                                </StackPanel>
                                </Label>
</StackPanel>