It turns out that not knowing may be better.
(this is an article discussing joint vs. separate accounts for couples)
How much do you need to know about how your spouse spends money? Maybe less than you think [The Conversaion] – “Joint accounts create transparency, and intuitively, transparency feels like a good thing in relationships. But I argue that some privacy is important even for highly committed couples – and money is no exception.”
Alas, there is no “correct answer for all couples,” just trends based on the data.