There are 2 or 3 common systems in place to ensure that one transmitter does not cause interference to others. The newest technology is 2.4 gHz transmitters. These have been out for a couple years. Typically these are found in systems costing several hundred dollars or more. With 2.4 gHz (brand names: JR Spectrum, Futaba FASST, etc.), the transmitters are designed to dynamically choose a frequency that is different from the others already in use at the time. Basically with 2.4 gHz, there is no worries.

The other popular technology is 72 mH transmitters. With these transmitters, each transmitter comes from the store with a predetermined frequency. There are about 50 unique frequencies. So you can interfere with each other if two people happen to show up at the same time on the same frequency. To avoid this, fields that are routinely used for RC planes adopt a frequency board where everyone checks in on their frequency.

At public parks (unmanaged places such as school soccer fields etc,), your best bet is the 2.4 gHz systems. This is certainly the way of the future.