How do we know there is no law making the average working American liable for income tax?

Every tax law must clearly and plainly identify three things:  The subject of the tax; the amount of the tax; and the persons or entities that are liable for the tax.  Even a cursory review of other tax laws, including all other federal tax laws, makes it obvious that these three elements of any tax law must be present.

The income tax law, however, is the only instance where there is no clear liability provision.  The only clear liability provision is § 1461, which assigns liability for the tax to those required to withhold taxes on Nonresident Aliens!  In fact, the term "Individual", as in "a tax is hereby imposed upon the taxable income of [married, single, etc.] individuals," is defined in 26 C.F.R. § 1.1441-1(c)(3) as a nonresident alien.

While 26 U.S.C. § 3403, which is not part of the income tax law, does require employers to withhold income tax owed by their employees, there is no law that makes the employee liable for the tax.  It is not by accident that the term "employee" applicable to § 3403 only includes government employees and officers (not employees) of corporations.

Thus, there is no law making the average working American liable for any income tax.