Talk to a probate attorney. They can tell you the easiest way to get the information.
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You will probably need to file an estate and get Letters Testamentary so you can get the account information from the banks and any other institution. Also, a good place to start is the IRS Office. you can request a transcript from them and see where all of his accounts and income is.
You can request the court to order a the person to provide an accounting of the assets of the deceased.