If someone is in jail -- or has been in prison in the past 25 years or so -- it's often possible to use online inmate locators to find out information about them, including what prison they are in, which crime they are imprisoned for, and how long of a sentence they are serving. Inmate locators often include numerous aliases for the person in question, so you may need to try different spellings of names you're unsure of.
Enter a first and last name or an inmate ID number into the Federal Bureau of Prisons Inmate Locator at http://www.bop.gov/iloc2/LocateInmate.jsp. When searching by name, an both first and last name are required and must be an exact match. The BOP Inmate Locator includes all persons who have been inmates at anytime from 1982 to present. It includes the names and records of inmates even after they have been released.
Search state prison lookups. It helps to have an idea of where the person was living prior to incarceration, so you can narrow your search to an appropriate geographic area.
Most lookup services are online and are searchable by name or ID number, but some, such as Florida Department of Corrections, have more extensive search tools, including eye color, height, age, sex, race, hair color, weight and other identifiers. Occasionally, however, you'll have to call or write to a prison system to inquire about an inmate they may be holding.
Check city or county inmate lookups. New York City's online inmate lookup, for example, searches by first and last name and date of birth, as well as by NYSID or book and case number.