You can use jgoodies validator interface to validate whatever input there is on that textfield Write your own validator class which implements the following interface. For instance the class name be TextFieldValidator.
Validator<T>
Precondition is that you bind your textfield to a PresentationModel.
TextFieldValidator<T> validator = (TextFieldValidator<T>) getPresentationModel().getValidator();
validator.setComponent(getYourTextField());
setComponent will set the JTextField reference in TextFieldValidator class and in the validate() method you can show pop ups/change background colors or what not.
Hope that helps.