§ 6201 – Assessment Authority
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(a) Authority of Secretary. The Secretary is authorized and required to make the inquiries, determinations, and assessments of all taxes imposed by this title, or accruing under any former internal revenue law, which have not been duly paid by stamp at the time and in the manner provided by law. Such authority shall extend to and include the following:
  
   (1) Taxes shown on return. The Secretary shall assess   
         all taxes determined by the taxpayer or by the secretary
         as to which returns or lists are made under this title.
   (2) Unpaid taxes payable by stamp.
        (A) Omitted stamps. ...
        (B) Check or Money Order not duly paid. ...