Use a ContentTemplate and determine whether the control contains content.

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<Canvas xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:c="CodeMapNS"
    x:Class="ContentControlSimple.Page1">
    <StackPanel>
        <StackPanel.Resources>
            <Style x:Key="ContentCtrl" TargetType="{x:Type ContentControl}">
                <Setter Property="Background" Value="Black"/>
                <Setter Property="Foreground" Value="Red"/>
                <Setter Property="FontSize" Value="40"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type ContentControl}">
                            <Grid>
                                <Ellipse Width="{TemplateBinding Width}" Height="{TemplateBinding Width}" Fill="{TemplateBinding Background}"/>
                                <ContentPresenter VerticalAlignment="Center"
                                    HorizontalAlignment="Center"/>
                            </Grid>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
            <DataTemplate x:Key="template1">
                <TextBlock Text="{Binding}" FontSize="12" FontWeight="Bold" TextWrapping="Wrap"></TextBlock>
            </DataTemplate>
        </StackPanel.Resources>
        <Label Margin="10, 10, 3, 3" Grid.Column="0" Grid.Row="2">
            <ContentControl Width="75" Style="{StaticResource ContentCtrl}" Content="Hello"/>
        </Label>
        <Label Margin="10, 10, 3, 3" Grid.Column="0" Grid.Row="3"
           Background="Blue">
            <ContentControl Name="contCtrl" ContentTemplate="{StaticResource template1}" 
          Content="This is the content of the content control."/>
        </Label>
        <Button Name="btn2" Margin="10, 10, 3, 3" Grid.Column="0" Grid.Row="4" Background="LightBlue" Click="OnClick">
            <TextBlock TextWrapping="Wrap">Click </TextBlock>
        </Button>

    </StackPanel>
</Canvas>
//File:Window.xaml.cs
using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Windows.Data;
using System.Windows.Media;
using System.Collections.ObjectModel;

namespace ContentControlSimple
{
    public partial class Page1 : Canvas
    {
        void OnClick(object sender, RoutedEventArgs e)
        {
            if (contCtrl.HasContent == true)
            {
                MessageBox.Show("contCtrl has content");
            }
        }
    }
}