Federal Inmate Locator
You can identify a federal inmate as they will always have five numbers, dash then three numbers 00000-000
Each state has different levels of information. Some states you will be able to see a mug shot of the inmate, race, eye color, height, weight, birth date, current facility, and release date. Whereas the worst of the inmate locators will only verify that the inmate is in prison.
There are many inmates listed in county jails where it may be difficult to verify information. We try and verify information the best we can. You can try a simple Google search and often get good results such as name of inmate, arrest, state. You may also try and google the name and address of a county jail and some will have inmate locators.