Robert in New Jersey,
What N2ZGE suggests above is valid, but he forget one other method availabe to those seeking out active frequencies - Signal Stalkers (or CloseCall, for those readers with Bearcats). Assuming your scanner is equipped with that capability, first try to verify that your target company is using two-way radios to begin with (see below); look for antennas on vehicles or portable radios hanging from hips. If your're using Signal Stalker (Radio Shack scanners), try to get a look at the antenna being used to narrow down the frequency band you're 'searching' through.
Do not be disappointed to find that any one (if not many) of the specific companies you are looking to monitor are not using regular two-way systems at all. In my hometown alone the local taxi service, many of the wreckers, and the security contractor for two nearby shopping centers all use a common carrier (AT&T Wireless or Verizon) or NexTels to communicate with their units. The security contractor in question was tricky, too. They purchased used police cars for their fleet and left the VHF-high band antennas that were mounted on them in place - leaving me for a while with the mistaken impression that they were using two-way radios. A closer inspection of a car only found an AM/FM radio in the dashboard. Interestingly enough that particular company made extensive use of a nearby UHF business band repeater in the past - before they switched to cell phones. Lastly, I've found that with information security being a cause for concern these days, taxis and tows are using cellular telephones (or similar services) more and more to talk to dispatch - especially when passing the credit card numbers of customers who opt to pay for the services provided them that way.
SCANdal