If I understand the question correctly, one can hear the control channel by either (1) turning on the TRX-1's "IF Output" function in Program Menu > Global Settings, or (2) programming the control channel as a conventional analog object.
Note that something must be plugged into the headphone jack for the IF Output function to work, even if listening through the speaker.
I find it useful to listen to control channels as a diagnostic tool when reception is difficult. Is it a clean sounding signal or one laden with cellular interference? Does it sound better with the attenuator on or off? Does repositioning the radio make a difference? Maximum strength on the signal meter alone could simply indicate maximum overload from a nearby cellular site.
A P25 LSM/CQPSK control channel signal has a very pronounced 4.8 KHz whine when monitored as an AM analog object. With a fully open squelch it can be heard even when the trunking signal is too weak for the TRX-1 to decode. So in a fringe area with no whine, there's no point in even trying to pick up the system. When I start hearing the whine, I give it a go.