I believe Unit ID decode is only possible with P25 systems. That's because the Radio ID number is a portion of the data stream. "Most of the time" this is associated with trunked systems.
There is a way of dealing with P25 on conventional channels/frequencies. To do that, one would program the system as a 1 frequency trunked system. I've tinkered with that and saw where unit IDs are decoded, but haven't messed with enough to be too knowledgeable other than it can be done.