In your form, create a method
ShowOutcome(bool success)
{
successLabel.Visible = success;
failureLabel.Visible = !success;
}
Call that method when your function knows whether it is a success or not.
[Edit]
To display neither label, you can create such a method:
ClearOutcome()
{
successLabel.Visible = failureLabel.Visible = false;
}
[/Edit]