Determine what you need to know. An effective and realistic crime novel will include details on crime procedurals and investigations, but they can also include everything from the types of accelerants arsonists prefer to use to how witnesses or suspects are interviewed. Determine how much you need to know and what you need to know before you begin writing. Any new information or details you discover during your research can drastically alter your plot or the characters in your novel.
Determine where to research. Some information, such as criminal law terminology, can be found at the library or online. But some information can only be gained from a live interview. For instance, detectives and/or criminals may speak in a lingo or slang that is specific to the locale of your novel's setting. This is information that you're not going to get from a book or movie.
Contact subjects for a live interview. Check your local government's website for contact information. You'll probably want to contact the public affairs department first. They'll help you set up interviews with experts in the field you're investigating.
Exploit whatever contacts you have in your own family or group of friends. For instance, if you have a family member or friend who is a DNA expert, a police officer, an attorney, or even a reformed (or active) criminal, interview him or her for research.
Ask the right questions. Know specifically what you'll need to know before you go into the interview. But don't be rigid. The best interviews are like conversations. Your subject might provide information you hadn't thought to ask about before.
If you're writing a police procedural, seek permission to attend a crime scene. Being on a live crime scene investigation will give you the kind of research you won't find in a book or online.
Attend court proceedings. Watching attorneys, judges, and juries in action will give you a clear idea of how trials are conducted. Court proceedings can include everything from arraignments to trials to sentencing hearings. Attending these proceedings can also put you in contact with lawyers, bailiffs, or any other officers of the court who can provide you with additional research for your novel.