First, amass all the information you know about the inmate. This includes their full name, aliases, date of birth, the date and place of their arrest, the date and place of their conviction, whether they were convicted on a state or federal crime, and, if they are currently incarcerated, their last known place of incarceration. These facts will be critical for helping you locate additional information about the inmate.
If the inmate is housed in a federal facility, much of their information will available from the federal Bureau of Prisons. Visit their website at www.bop.gov. This will allow you to search by name or by the inmates' registration number. It will provide you the inmate's full name, age, current location, registration number and their release date.
For inmate residing in certain state and county prisons, various records are similarly available on the internet. (New York state, for instance, allows you to look up inmate information 24 hours a day.) Using what you know of where the inmate was arrested and convicted, visit the appropriate websites and perform searches through their systems using different criteria, such as name, age, sex, race and the date of conviction. A list of these websites is available at http://www.theinmatelocator.com.
However, many of these local areas have not yet placed their records online, meaning that this information can only be obtained by calling or visiting judicial or correctional facilities, such as courthouses, jails and state prisons. Either look up rules regarding the release of these records online, or call these facilities to request information about their policies.