How to trigger all Data errors on WPF MVVM form

Question

I am trying to show 'Required' in red text when the form is loaded and the text is empty what is the best way to display all required field once the form is visible?

I have about 7 more textboxes.

XAML:

<TextBox x:Name="SubTotalApples" Text="{Binding SubTotalApplesValue, StringFormat={}{0:0.00##}, ValidatesOnDataErrors=True, UpdateSourceTrigger=LostFocus}"/>

<TextBox x:Name="SubTotalOranges" Text="{Binding SubTotalOrangesValue, StringFormat={}{0:0.00##}, ValidatesOnDataErrors=True, UpdateSourceTrigger=LostFocus}/>

ViewModels properties

[Required(ErrorMessage = "Required")]
public bool? SubTotalApplesValue 


[Required(ErrorMessage = "Required")]
public bool? SubTotalOrangesValue