How do you determine which validator failed?

Credit to Steven for this answer, but I had to make some changes for it to work as this.Validators.Where() had some problems.

using System.Linq;List<IValidator> errored = this.Validators.Cast<IValidator>().Where(v => !v.IsValid).ToList();

In code (page_load), you can do this:
(per MSDN: http://msdn.microsoft.com/en-US/library/dh9ad08f%28v=VS.80%29.aspx)

If (Me.IsPostBack) Then    Me.Validate()    If (Not Me.IsValid) Then        Dim msg As String        ' Loop through all validation controls to see which         ' generated the error(s).        Dim oValidator As IValidator        For Each oValidator In Validators            If oValidator.IsValid = False Then                msg = msg & "<br />" & oValidator.ErrorMessage            End If        Next        Label1.Text = msg    End IfEnd If

In the markup, you can...

• You can put "text" on your validator (like anasterisk...)
• Or use a validation_summary control (which requires an error message on your validator)...

The accepted answer allows you to find the validation message of the validator that failed. If you want to find the ID of the control that failed validation, it is possible to get that by casting the validator to a BaseValidator which exposes the ControlToValidate property. For example:

For Each v As BaseValidator In Page.Validators    If Not v.IsValid Then        ' You can see the control to validate name and error message here.        Debug.WriteLine(v.ControlToValidate)        Debug.WriteLine(v.ErrorMessage)    End IfNext