How can I obtain the background information a private detective would find?

Detectives have a wide variety of techniques available in conducting investigations. However, the majority of cases are solved by the interrogation of suspects and the interviewing of witnesses, which takes time. Besides interrogations, detectives may rely on a network of informants they have cultivated over the years. Informants often have connections with persons a detective would not be able to approach formally. Evidence collection and preservation can also help in identifying a potential suspect(s).