The easiest way to find out which jail a person is in is usually to use VINELink.com, an online court and inmate records search provided by the National Victim Notification Network. If you have an idea of where the person was arrested, you can also search local jails directly.
VINELink is connected to jail records in most states, but it does not work in South Dakota, Kansas and Maine. It has limited information in Arizona, Montana, Alabama, Georgia, Massachusetts, West Virginia and New Hampshire. However, even in those states it is worth a try.
If VINELink is unhelpful, AncestorHunt.com provides links to individual state and county jail searches. Most city and county jails are online and can be searched directly. You can also call each establishment individually. This may be more efficient if you have an idea of where the person was arrested, because there is usually only one county and one city jail.
Jail is for temporary detention as a person awaits trial and sentencing. It can also be used for shorter sentences, usually less than one year. If you are looking for an inmate who has been sentenced to a longer term of incarceration, you may need to look at state and federal prisons. Use the Federal Bureau of Prisons' Inmate Locator to find an inmate in the prison system.