How to find out if my deceased husband was hiding money from me?

I’ve found information on how to find out if you’re spouse is hiding money from you if you’re going through a divorce but can’t seem to find anything about a deceased spouse.

My husband died a year and a half ago. The marriage was not a good one, and I’m pretty sure he cheated on me while he was alive and I’m also pretty sure he had secret bank accounts I didn’t know about. I doubt that the banks would tell me that information.

Anyone know how to go about finding this out? I know I can hire a private detective, but from what I can tell, that would cost me a fair amount.

Any tips would be helpful. Thanks!!