Read the uncodified law - the chapter or session law - to trace back the language of the law and determine how each amendment affected the law.
California session laws are published in Statutes and Amendments to the Codes ( KFC25 - 4th Floor Research Alcove). The library has the print version of Statutes and Amendments to the Codes for the years 1849 through 2008.
For chapter laws enacted since 1994, you can find the bill number in the Credits that follow the text of the code section on Westlaw.
For older enactments, you will need to use the chapter number to find the bill number. The Summary Digest ( KFC 5 - 5th Floor Stacks) is a good place to locate the bill number, a summary of the bill’s text, and its effective date. Beginning in 1968, the Summary Digest for each year is also available in the last volume of the Statutes and Amendments to the Codes.
In addition, the first volume of every year's Statutes and Amendments to the Code has a chart called the Table of Laws Enacted, which also has a conversion chart for chapter number to bill number.
The Summary Digest [ KFC 5 - 5th Floor Stacks] is available at UC Hastings for 1949-51 & 1960-2008. The Summary Digest is available at the San Francisco Public Library for most of the years from 1937-2008.