That can be tricky.
Your best bet would be to use a program like Unitrunker, which is an extremely powerful tool for analyzing trunked radio systems. From there you can see the activity and the LCN numbers, but NOT the frequencies. You will have to manually find those. The way I do it is either look for the FCC license for the system in question, and then put all those frequencies from the license in a scanner. While watching Unitrunker, when a call comes in note which LCN its on, and then see what frequency the scanner stopped on.
You can get lucky after a few hits and maybe notice a pattern to the numbering. (one system near me uses 5 frequencies for LCN 1-5 from 856.5875, 857.5875 thru 860.5875) The unlucky alternative if you have no idea what you are hearing is the same method, but set your scanner to search the whole band from 851 to 862, and hope they have enough activity to let you find them before they move. (a dead giveaway for and EDACS channel is at the end of every analog voice call there is a series of loud BEEP BEEP BEEP that lasts 2-3 seconds, this will be a good indicator you found an EDACS working channel)
Hope that helps, there is a ton of resources over at the
Trunking Control Channel Decoding - The RadioReference.com Forums forum here.