When a statute is passed by Congress, it is assigned a public law number (e.g. Pub. L. 93-406) and published by date in the official Statutes at Large (Stat.).
It is then organized by topic in the official United States Code (U.S.C.). Westlaw and Lexis take the basic text of the U.S.C. and add annotations (notes explaining each statute's history and identifying relevant cases and secondary sources interpreting the statute) to produce Westlaw's unofficial United States Code Annotated (U.S.C.A.) and Lexis' unofficial United States Code Service (U.S.C.S.).
The easiest way to view a statute's history is to type the statute's citation into the main Lexis or Westlaw search box and then hit enter. For example, if you are interested in researching the history of 29 U.S.C. § 1182, type 29 USC 1182 in the main Lexis or Westlaw search box and hit enter.
After the text of a statute, you will see a citation to the public law that created the statute, followed by citations to each public law that amended it.
To help you identify which public law is relevant to your research, both Lexis and Westlaw provide notes explaining what each amendment did. These are shown just after the list of the public law citations on Lexis and under the History > Editor's and Revisor's Notes subtab on Westlaw.
If you do not have access to Lexis or Westlaw, you can access the U.S.C.S. on Nexis Uni or in print at the law library and you can access the U.S.C.A. in print at the law library:
Alternatively, you can retrieve the statute in one of the free online versions of the official United States Code (U.S.C.). After the text of each statute, the U.S.C. provides a citation to the public law that created the statute, followed by citations to each public law that amended it, followed by notes explaining what each amendment did. The text and basic history information for the statute will be the same as that found in the U.S.C.A. and U.S.C.S. but the free versions will not provide as detailed an explanation of what each amendment did.
The following free online versions of the U.S.C. are generally trustworthy: