While it is difficult in general to ascertain whether a foreign word or phrase used in an utterance is borrowed or just an instance of code-mixing , one tell tale sign is that only proficient multilinguals can code-mix, while even monolingual speakers can use borrowed words because, by definition, these are part of the vocabulary of a language.
just because an English speaker understands and uses the word "tortilla" does not imply that she can speak or understand Spanish.