Several resources will let you know what questions were asked in the Decennial Censuses and other specific surveys:
Historical Statistics of the United States
Use this online resource to find historical statistics from Colonial times to the present
U.S. Census
A tremendous amount of data about the U.S., current and historical, is available through the U.S. Census. However it's not always easy to search. The best guide I've seen to using that data is from a librarian at UC Berkeley: http://guides.lib.berkeley.edu/USCensus/Intro
Community Facts
Quickly find the most current demographic information for a specific city, county, or even zip code anywhere in the U.S. using the guided search. Generally 5 years of data is immediately available.
California Statistics
A wide variety of California statistics, collected by state and local sources can be found at this site.