Thanks to the Internet, it's easy to look up any type of information. From census records, to marriage certificates and military records, information about it can be obtained online. If you are looking to find out if you are of Jewish descent, there are a few things you can do, both online and off, to discover your background.
Talk to your mother's family. For centuries, Judaism has been passed on through matrilineal descent. This means, if your mother was Jewish, you are considered Jewish. However, if your father is Jewish, that does not necessarily make you Jewish. Inquire of family about your mother's religious and ethnic identity as well as that of other women in the family who are directly related your mother, such as her mother and grandmother.
Contact a genealogy-driven DNA testing organization. Organizations like FamilyTreeDNA use cheek-swab DNA testing to identify a person's genetic haplotype, a distinct genetic pattern within chromosomes, to connect the person with an indigenous group or tribe.
Look through family obituaries. Obituaries have a wealth of information concerning genealogy. You can use the information from the obituary to research other people within your family unit, search surnames, dates of birth and death, and places of burial. Find out if funerals where held at synagogues and if surnames hint of Jewish heritage.
Visit or research cemeteries where your family is buried. Since cremation is against Jewish tradition, cemetery records can serve as a thorough resource for information on your genealogy. People who are Jewish are most often buried in Jewish cemeteries. Some cemeteries have databases with names, birth dates and photos of the deceased.
Check the Consolidated Jewish Surname Index. According to Avotaynu, the leading publisher of books on Jewish genealogy, the index cross references close to 699,084 surnames, most of which are Jewish, against databases that "combined contain more than 7.3 million records of Jewish descent."