When a person is brought to a jail or a prison, that person is given an inmate booking number. The inmate booking number is essentially a way to record the individual's information and serves as a way to identify the individual. Typically, it is located in the booking report. You can also find the information by entering the incarcerated individual's name into an inmate locator search engine, if there is one on the website for the specific correctional institution where the individual is housed.
Check the booking report filled out by the staff at the correctional institution where the inmate is being housed. Typically, the booking number will be listed in a box named "inmate number" or "booking number."
Call the correctional institution where the person is or was being housed. If you know where the individual was incarcerated, you can call that institution and request the booking number. Typically, you will have to give the individual's first and last name. In some cases, you may need to provide additional information; for example, the California Department of Corrections and Rehabilitation requires the individual's name and birth date.
Use an inmate locator on the internet. Many prisons and jails provide a search engine for locating inmates at that particular correctional institution. Visit the website of the prison where the individual is being housed and look for a tab or link that says "search for inmates," or something similar. Fill in the required information (typically the name of the inmate). The search should return the inmate's booking number.