Assets that are not included in a probate include assets held in a revocable trust, assets that pass by beneficiary designation (life insurance/retirement plans, etc), and assets held in joint tenancy with right of survivorship or other pay on death on death form.
The IRS won't have possession of these assets. For real property, you can check the county records to see how title is held.
You really need to meet with an attorney who can provide more specific advice to you based on your particular facts and what you are seeking to accomplish. That is not clear from your question.
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Attorney Symons has given you a good answer. Aside from assets that are part of the public record, like real estate, it is going to be next to impossible for you to determine what passed outside of probate. You will not be able to get information from banks and other financial institutions, outside of a lawsuit setting. Privacy laws prevent companies from divulging information on assets passing outside probate. The same thing is true with a trust, unless you are one of the beneficiaries.
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As noted by Ms. Symons' answers, it appears you are trying to uncover details about a probate matter and that you are either a beneficiary or contingent beneficiary that believes has an interest in the estate. It is commendable that you are seeking answers in the probate code and online through AVVO but it is extremely advisable that you consult with an attorney that is experienced in the area and discuss the details of your particular matter so that you do not discover what you should or could have done when it is too late to do anything about it.
Joint tenancy Rights of survivorship Inheriting property Real estate Privacy law Wills and estates Estates Inheritance rights Estate assets Distribution of estate assets Estate property Rights of survivorship and estate planning Taxes and estate planning Wills Executor of will Probate Probate assets Types of trusts Revocable trust Tax law