In many parts of the Western world, notably the United States, information such as a person's date of birth, address and residence is public record. This means that anyone conducting a background check on a person for any reason can access very basic records on the individual, usually for free, to determine he or she is who he or she appears to be. This is particularly important when interviewing someone for a job or investigating a suspected criminal. While some information such as actual criminal records is harder to come by, finding out a person's date of birth is not as difficult.
Search for the person on Google. Google is your basic research tool, used by everyone around the world. Even national and international security agencies use Google. You may discover the person has a personal website that lists their date of birth or find photos of one of his or her birthdaysthat someone may have posted online. This may not give you the person's actual date of birth, but it will give you an estimate of how old the person is so that you can expand your search elsewhere.
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Try to find that person's page on a social networking site. She may have listed her birth date. Note that this is not entirely accurate either, but if the person has many friends linked to his account, it's more probable that the date is correct, as such websites publish users' birthdays among friends lists and the person would likely want their friends to know their correct date of birth.
Search for the person on Lexis-Nexis, a powerful database that is often accessible from public libraries and universities. Fill out as much information you already know about the person, such as their home town, names of relatives or middle name or initial to ensure better results.
Locate the person on a public telephone and address directory such as Zabasearch and Intelius. It isn't guaranteed, but search listings can often contain a date of birth--or at least a month and year of birth.
Call and then file a written request to the division of vital records of the state you know the person was born in to release a copy of his or her birth certificate. Each state has its own rules on how to do this and some may give you a hard time about it. This process also tends to take longer.
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You may have to be a licensed private investigator to go through vital records if the person is still alive. If you are are a family member doing genealogical research, you may be able to access the birth certificate through the vital records office.