Enumerators of the 1900 census were instructed to record the names of every person in the household. Enumerators were asked to include the following categories in the census: name; address; relationship to the head of household; color or race; sex; month and year of birth; age at last birthday; marital status; number of years married; the total number of children born of the mother; the number of those children living; places of birth of each individual and the parents of each individual; if the individual was foreign born, the year of immigration and the number of years in the United States; the citizenship status of foreign-born individuals over age twenty-one; occupation; whether the person could read, write, and speak English; whether the home was owned or rented; whether the home was on a farm; and whether the home was mortgaged. The categories allowed Congress to determine persons residing in the United States for collection of taxes and the appropriation of seats in the House of Representatives.
The official enumeration day of the 1900 census was 1 June 1900. All questions asked were supposed to refer to that date. By 1900, there were a total of forty-five states in the Union, with Utah being the latest addition and Alaska, Arizona, Hawaii, New Mexico, and Oklahoma enumerated as territories.
Taken from Szucs, Loretto Dennis, "Research in Census Records." In The Source: A Guidebook of American Genealogy, ed. Loretto Dennis Szucs and Sandra Hargreaves Luebking (Salt Lake City: Ancestry, 1997).
In-person microfilm inspection
Basic information on each household in the 1900 census was transferred to cards that were then alphabetized using soundex codes and microfilmed. Use a finding guide to find the right film roll containing the soundex code for the name you seek in the state you're searching. Each card has the enumeration district and page number of the original information, which is used to determine which microfilm roll contains the relevant census return.
FamilySearch, Ancestry, and Heritage Quest have all indexed the 1900 census and connected the index to images of the original microfilm. FamilySearch permits free searching of nearly the entire 1900 census, but the search interface is more limited than Ancestry's (it does not permit searching only within a particular county, or for fields like race, gender, or parents' birthplace). The results display, however, is more rebust, immediately displaying birth month, spouse, parents and children. Heritage Quest permits searching only by name, place, age range, gender and race, and its results display is similarly limited.