get a frequency scanner.. our club has one they are $100 the one we have scans all the 72mhz freqecys and tells you what channel everyone is on that has a transmitter onwe usually have so few people at our field there is no problem. but we still scan it regularly. we have also used an IFR spectrum analyzer just to make sure there were no harmonics spikeing from other frequencys since we fly near a mountain that is rented by people with repeaters. all is clear