I have the suspicion that my SBXW has opened a checking account in her name (alone) and that she uses it to deposit all of her pay increase (she claims that she had no salary increase last year, but her W-2 shows considerably more than the previous one. Is there a way to find out if that is true or in general find out about hidden finances?

We used to share all this info, but during the last year, she also opened a CC account in her own name (all the other accounts were common) ; and the amounts charged to this cc are relatively large (she pays it from our common checking account) but the cc statements are all electronic so I can't see what is going on; when I asked, she simply made up some BS about kids expenses but didn't want to share receipts or statements.