The following post is copied from the "Unethical Life Pro Tips" subreddit - it was not written by me. I've copied the body of the post, in case it gets deleted:
Hey everyone. I'm a police dispatcher and I wanted to share some tips because my boss is a dick. Pretty much every police department uses radios to communicate, and not many agencies use encrypted systems.
"How do I get set up?"
If you live in a somewhat large city, chances are there might be a live feed of your area's radio traffic already being streamed. In that case, all you need is an app on your phone.
If a live stream isn't available, you can get a radio scanner for about $150 (for an analog system) to $300 (for newer digital systems). Not sure which one you need? Go to Radio Reference and put your zip code in the search box. It will give you a list of all the public safety channels in your area. Click on the name of the agency, and look under "Mode". If it has "A" it's analog, "D" is digital, and "M" is mixed (use a digital scanner). If you see the letter "E", that means that channel is encrypted and a scanner won't work (see below for a workaround). A digital radio can also usually pick up analog signals as well. There are programming guides online, but many new scanners are plug and play: you just punch in your zip code and it programs itself.
"So what can I do with a scanner?"
Lots of things, ranging from harmless to borderline-illegal:
Listen out for traffic accidents or checkpoints and navigate around them.
Set your radio in "close call" mode and it will chirp at any strong signals nearby. That means there's generally a cop, paramedic, or firefighter nearby.
Get the scoop on what's going on in the neighborhood.
Use it when committing a crime. You can hear the police get dispatched, so you know when it's time to leave. You can also hear if they're going to a call on the other side of town, so use that to your advantage.
"I hear them talking, but I can't understand them. What do I do?"
If it's just them talking too fast, you'll get used to it. Trust me. It's easy to develop a "radio ear" in as little as a few hours of listening.
Many departments use "10-codes" which are used to shorten messages and provide some (very weak) security. Many of you already know a few, such as 10-4 for "understood". 10-4 is the only universal 10-code, but there are some semi-standardized lists you can find here. If those codes aren't the same ones used near you, look up 10 codes for the closest large city or state police department, they tend to be similar to local 10-codes.
You don't even need to know what the 10-codes are for certain crimes. When a call is dispatched, the address is almost always given in an uncoded format. For example, we might say:
All units, 10-35 in progress at 123 Main Street. Responding units acknowledge.
So if you're breaking into the house at 123 Main Street, you don't need to know that a 10-35 is a burglary, you just know that the cops are on their way and it's time to get out of there. Also listen for alert tones (such as a long, steady beep or a "wee-woo" warble tone) that often indicate a serious crime in progress.
If you hear static or can't understand much at all, you might need a good antenna. Amazon sells a decent car roof antenna for about $15 and it's well worth it. Range depends on the antenna you get and the system they're using, but for me is generally about 20 miles from the nearest repeater.
My local department encrypts their radio channels. What can I do?"
An encrypted system isn't the end of the world. If you plan on using a radio for... less than legal reasons, then it's easy to steal one. Call the cops for a routine theft report or something, and have someone else steal the handheld radio from the car. Dispatch can remotely deactivate a radio, but we very rarely do unless you transmit on it and disrupt our operations. The NYPD has been having issues with stolen radios and they don't even encrypt most of their channels.
Anything I should be careful about?
A few states (such as Florida and New York) prohibit the use of a scanner in a vehicle, unless you're a licensed amateur radio operator (which is an easy license to get).
Using a scanner during a crime can get you extra charges, depending on where you live.
Dispatch has no way of knowing if you're using a scanner, but officers will often let us know if there are scanners located at an address so we can update our records.
There are other ways for officers and dispatch to communicate. They will often relay sensitive information (such as a criminal history) via phone or an instant messenger system.
TL;DR Get a police radio scanner. They have many uses if you want to hear what the cops are doing.
Also, if anyone needs help or has questions, feel free to PM me.
EDIT: I'd recommend against stealing a radio. Because, you know, stealing is bad. If it's encrypted, just take that as a sign that maybe they don't want you listening.
Note that one person in the reddit comments claimed that stealing a scanner from a police car is a very bad idea, because "many metro departments can gps locate individual handsets (primarily for emergency backup dispatching)." So, make sure they can't do that if you decide to steal one.