1. Look at the trucks and stuff. Do the Garden City DPW vehicles have antennas on them? This can be a clue. What frequency band (VHF, Low-Band, UHF, etc) do the antennas look to be for.
2. Are you listening to all the frequencies listed to Garden City? Don't neglect the old PD and FD VHF or UHF freqs. Some cities are using those for DPW and parks and recreation after their fire and police go to 800 Mhz, since the radio equipment is already bought and available.
3. Are DPW workers carrying handhelds. Look at the antenna on the radio. Fiqure out how to tell if it is a VHF, UHF, or 800 mhz radio antenna. Hang out at the Big Boys at Ford and Harrison while they are eating lunch and look at the radios, if they carry them.
4. If they are carrying Nextels, more than likely they are exclusively using Nextel, and no longer use conventional two way.
Many, many years ago, the Garden City DPW was on 45 MHz, and they had long quarter-wave antennas on their vehicles. If you spot a long antenna that looks like a CB radio would use, then its very possible they are still on that band, but operating with an expired license. Westland DPW did the same thing for many years on 45.12 MHz