Use textbox events like change, keyup, keydown, keypress events ...

 private bool CheckZeroValue(Panel Container1)
        {
            foreach (Control control in Container1.Controls)
            {
                if (control is TextBox)
                {
                    TextBox textBox = (control as TextBox);
                    if (string.IsNullOrEmpty(textBox.Text.Trim()))
                    {
                        throw new Exception("Either Null or Empty !");
                        textBox.Focus();
                        return false;
                    }
                }
            }
            return true;