Once you find a frequency, look it up here: http://wireless2.fcc.gov/UlsApp/UlsSearch/searchAdvanced.jsp
To see who, in reasonable receiving range of your scanner, is licensed for that freq.
If you have a radio that scans PL tones too determine the PL (if any) being used.
For example in my area police and fire dept's may share freq's: my town and another, ~25-30mi away share a UHF frequency, but they use differant PL's. The local signals are *much* stronger than the distant ones so the local signal easily captures the receiver if both are active at once, otherwise one's signal won't open the tone squelch of the other, but a carrier squelched scanner can hear either, and will hear the nearest it both are on. If you can't ID where they are, Googling for the Licensee, freq and PL will often resolve the question.