This is a good and important question, and there is no easy way to answer it in the abstract. At a general level, any defense will go to attacking one of the “elements” of the crime. In the case of attempted tax evasion, for example, the IRS must prove three elements beyond a reasonable doubt: that you owed tax, acted willfully, and you made an affirmative act of evasion. Thus, a legal defense against an evasion charge will seek to deny one or more of these in various elements.
One of the methods of presenting a defense is to discredit the evidence offered against you, either because it is false or, even if it is true, try and find an argument to render it “inadmissible.” Technical rules exist regarding when evidence is admissible at trial. Often times these rules bend in favor of the taxpayer. For example, in one case, the IRS alleged that the taxpayer committed an “affirmative act” of evasion before the date of his tax deficiency. The indictment, however, only mentioned that the “crime date” was the date of the deficiency—the IRS failed to mention the pre-deficiency period. Thus, because the government made a technical error with the indictment, the IRS was precluded from using any evidence during the time prior to the deficiency. As a result, the taxpayer had a stronger defense. United States v. Voight, 89 F.3d 1050, 1089-90 (3d Cir.), cert. denied, 519 U.S. 1047 (1996). Qualified legal counsel can help you discern whether the IRS has made a technical error like this in your favor. In many cases, it can be the deciding factor in a case.
