this depends on your circumstances. Can you tell me more?
Dear Ms MB,
I see you have not responded to my request for additional information. So, let me say this.
If you suspect tax fraud you would report it to the IRS at the following address. The IRS will investigate it anonymously. If you provide the IRS your name and contact information, then you may be eligible for an award, and they will still keep your name private.
If this is a case where you are trying to prove tax fraud involving a business parter or activity in which you are involved, then you would have to hire an external auditor to conduct an audit, and to request IRS certified copies of the tax return for the business in order to compare and contrast the audit report and the return.