It is much easier to find out if someone is in jail than why. Privacy laws and the lack of centralized information make it challenging to find criminal charges on prisoners. Each state has its own public access laws for records held by its law enforcement, courts and law enforcement agencies. In most states, criminal records for living individuals cannot be accessed without their written consent unless public disclosure rules, such as Megan's Law, provide otherwise.
Find out where the prisoner is incarcerated. Friendsbeyondthewall.com combines inmate locators for state and federal prisoners in one place.
Search the public information tab in the Department of Corrections (DOC) website for the state housing the prisoner. Most states will only provide the location and scheduled release date of the prisoner. In Alabama, you can find out if someone is on death row. In Montana, you can find a list of escaped prisoners with their rap sheets. West Virginia provides sentencing information with the disclaimer that it may or may not reflect the nature of the crime.
Review local newspapers for arrest records. You can find criminal charges on a prisoner by looking at public notice sections of newspapers where the prisoner resided before their incarceration.