The best way to determine scanner frequencies and codes for different cities is to get in contact with the local ham radio club and other amateur radio enthusiasts. The amateur radio community has compiled extensive lists of channels and is an excellent resource for those interested in local broadcasts.
When trying to determine local frequencies without outside help, the best place to start is by getting ham radio equipment that is capable of high frequency, very high frequency and ultra high frequency communication. This is due to the fact that local channels, which can be used for police, fire, emergency, maintenance, business and private purposes, can cover a wide variety of "bands," or sections of the radio spectrum as divided by the Federal Communications Commission. The FCC divided up frequency allocations to these different uses, and publishes the guidelines with regular updates.
Using their information is an excellent way to determine frequencies and codes, especially when trying to find frequencies associated with specific uses. Studying for the technician class ham radio license provides the knowledge necessary to explore the airwaves safely. Passing the technician class ham radio exam, and earning a license, makes broadcasting legal in the bands allocated to amateur radio.