Are you currently in litigation? Under what circumstances would you like to request that an employed be asked to "prove their payments"?
In litigation, tax information is not necessarily "off limits." If the tax forms you are requesting (Forms 1099s, Forms W-2, for example) would tend to make a fact in dispute more of less probable than the fact would be without the forms and the fact is "of consequence" in determining the action, the forms are relevant, and generally admissible. If the employer is the opposing party in litigation, the Forms may not be hearsay (see Federal Rule of Evidence 801(d)).
This response is for general informational purposes only. It does not create or imply an attorney-client relationship between the reader or any other person and Lapekas Law, P.A.. This response may not be relied upon to avoid civil penalties that may be imposed by the IRS regarding any matter discussed herein.
The information and supporting documents you are entitled to will depend, to a very large extent, on the reason for needing it. You can always ask but an objection might be interposed.
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In State (California) court litigation the discovery of tax returns is generally prohibited. Two obvious exceptions: family law court and tax refund litigation. In Federal court litigation tax returns are generally discoverable by the parties to the lawsuit.