I would check all the licensed frequencies for a control channel, and then you could simply create a new P25 system in your SDS100, if you find a control.
DSD+ is a good program for analyzing systems, but the SDS100 can show many system parameters, like system ID, NAC, WACN, etc.
There appears to be some kind of signal on 155.6400, but it's not constant, so likely not a control channel. From near Fredericksburg, I wasn't able to get DSD+ to give me any kind of useful information. The signal might simply be too weak.