Clarify to the best of your ability that a crime is being committed. As tax evasion is a serious crime, and an investigation can be quite an annoyance for those suspected, so even an innocent business would prefer to not have to go through it. In addition, the Internal Revenue Service (IRS) asks you to give your reasons for suspicion, and most other organizations will as well. Try to make sure your suspicions are well founded before contacting the government.
For federal tax evasion in the United States, go to the website of the IRS, and follow the clear directions for reporting fraud. According to the website, you can "use Form 3949-A, [or] you may send a letter" to the organization directly. The IRS offers clear and detailed procedures for doing this on the same site.
Tax evasion related to state taxes is reported differently depending on each individual state, but usually it is easy to report such crimes. New York state, for example, offers a simple hotline as well as an online form for citizens to help "make sure all businesses and individuals pay their fair share." The revenue collection agency for the the respective state of the suspected tax cheat should offer its own simple process for reporting this pervasive crime.
For local taxes, such as counties and towns, contact your local government to find out its procedure. Usually, it will take little more than a phone call for an investigation to begin, as most jurisdictions depend on tax revenue to keep services and infrastructure running, and finding those who evade taxes is crucial to maintaining this revenue. Usually, contacting the town hall or any other center of government will be sufficient.
If you prefer to remain anonymous or have your identity kept confidential, inform the revenue collection agency upon reporting the suspected tax evasion. In many instances, they are happy to do so, and this can help protect yourself against unwanted vengeance by the business in question.