Many are using simplex systems, which means that they transmit and recieve on the same frequency, so you should have good luck finding one frequency where you may hear both sides of the conversation.
Some places also have a frequency for talking between employees.
This is reportedly where a wealth of interesting transmissions can be found. Plus you get to find out what good coupons or specials are going on without actually being there. This is not a complete list. This is a guide for you to get an idea of where you should be using the search command on your scanner. One of the microphones is either always on, or tripped by a car driving up.