So is there a mens rea involved here? A ‘state of mind’ or ‘state of intention’ defect that would cause the marriage to be invalid?
Is there some sort of guideline a Catholic can turn to in order to help him or herself decide whether what they are in is a ‘false marriage’ or whether they are in a valid marriage that is essentially in a state of ‘breach’, as it were - a breaking or non-honoring of vows?
That indeed could be one reason to declare that a marriage is invalid. Again, this is a question that the marriage was invalid from its inception, not that a valid marriage does not continue to exist.
Is there some sort of guideline a Catholic can turn to in order to help him or herself decide whether what they are in is a ‘false marriage’ or whether they are in a valid marriage that is essentially in a state of ‘breach’, as it were - a breaking or non-honoring of vows?
The presumption is that a marriage is valid. For it to be declared invalid one must approach church tribunal with evidence that it is invalid. There would then be a judicial process to ascertain the truth. It is not up to the individual to make such a decision. We are all prone to delude ourselves and there is also the possibility that the spouses would disagree. But the first step would be to talk to one's parish priest to discuss the possible reasons that a marriage would be invalid.
Yes, but you won't find it in a book. If you're looking for "written" guidelines, there are none. You have to apply for an "annulment" and see what the bureaucrats say about your "application".
My best advice, is stay close to God in service and prayer, and your 'heart' will lead you to the right decision.