There's an obvious problem here: you say that you, as a private individual, wish to monitor another adult's phone without their knowledge, which naturally implies without their consent. That would be illegal in the USA (where this forum is based) and in the UK (where I am), and being married to the person makes no difference to the legality, nor does owning the phone. Since we don't want to cause legal trouble for the forum, posting advice on how to commit what, as you describe it, appears to be an illegal act in the jurisdiction where the forum is based is obviously problematic.
But actually it would make no real difference what reason you gave or who you said you wished to monitor. One reason is that we've no way of knowing who you actually are and what your real intention is - don't take that personally, it's simply a statement of fact about the internet: any of us could be someone different from who we present ourselves as online. But secondly, even if you could prove you had good intentions and were operating strictly within the law, any answer we post here can easily be found by any domestic abuser, stalker or other criminal who can use a search engine, and hence it would be irresponsible to post such information.
So sorry, I'm afraid I can't help you.