Check if a file is selected with FileUpload

How can i check if there is a file selected...
I think its possible before the user presses a button ? (also it should be because its in a wizard item... ).

But the code below works.. he checks so i can push the button but soon he returns and empty's the fileupload and i get the error that i should fill this item...
How to change?

<asp:FileUpload ID="fuAfbeelding" runat="server" />
                               <asp:RequiredFieldValidator ID="rAfbeelding" runat="server"
                                ControlToValidate="fuAfbeelding" ErrorMessage=" U dient een afbeelding in te ...