Here is a good example for FCK editor validation.
Declare a Custom Validator;
<asp:CustomValidator runat="server" ID="cvBody" SetFocusOnError="true" Display="dynamic" Text="The Body is required" ClientValidationFunction="ValidateContentText"></asp:CustomValidator>
Add this Javascript function to your page
function ValidateContentText(source,args)
{
var fckBody= FCKeditorAPI.GetInstance('<%=FCKeditor1.ClientID %>');
args.IsValid = fckBody.GetXHTML(true) != "";
}
Ref: “fck validation“