The most obvious way, would be an audit of the persons tax return (if filed) or inquire into why they are not filing. If they are "self employed" and can only prove income with bank deposits the IRS is going to ask for an explanation of the deposits or income and the source. The source won't be anonymous for long....
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The biggest tool for the IRS is going after unreported income. When you get audited, it is the taxpayers burden to prove everything. Your word is not proof. Alleging people anonymously gave to you out of the goodness of their hearts wont cut it. You're not a charity. If you cannot establish who gave you the gift, and they cannot confirm the source, they will just access it as income.
The general advice above does not constitute an attorney-client relationship: you haven't hired me or my firm or given me confidential information by posting on this public forum, and my answer on this public forum does not constitute attorney-client advice. IRS Circular 230 Disclosure: In order to comply with requirements imposed by the Internal Revenue Service, we inform you that any U.S. tax advice contained in this communication (including any attachments) is not intended to be used, and cannot be used, for the purpose of (i) avoiding penalties under the Internal Revenue Code or (ii) promoting, marketing, or recommending to another party any transaction or matter addressed herein. While I am licensed to practice in New York and California, I do not actively practice in New York. Regardless, nothing said should be deemed an opinion of law of any state. All readers need to do their own research or pay an attorney for a legal opinion if one is necessary or desired.
Everything you receive under code section 61 is income, unless you can prove otherwise. Gifts are one of the exceptions, but it is the taxpayer's burden to prove the item is actually a gift. This would not be possible under the facts you present. This is what caused Al Capone to go to jail. He was convicted of tax evasion and not for his other crimes.
H. Daniel Lively, Esq., LL.M., CPA Certified Tax Specialist, CA Board of Legal Specialization email@example.com www.USTaxRescue.com 714-708-2593 Mr. Lively is a Certified Tax Specialist by the State Bar of California Board of Legal Specialization. He can be reached at 714-708-2593 or USTaxRescue.com.Any individual seeking legal advice for their own situation should retain their own legal counsel as this response provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this response cannot be used to eliminate penalties with the IRS or any other governmental agency.