While the Audio Box does not have a channel for signal strength testing, the 18R2 Controller reports signal strength on channel 99.
Power on the 18R2 and when it finishes the startup sequence, change the channel to 99 (firmware version 5.1 or lower) or 199 (firmware version 6.0 or higher). Press and release the SYNC button to display the Audio Box signal strength. It's recommended to have a signal value between 0 and -75. However, note that the Audio Box only requires one "heartbeat" command from the 18R2 (sent once a second) to begin the audio; therefore, it is much more resilient to a weaker signal as only one received command is required to play the audio.
If you have multiple Audio Boxes synced to the 18R2, you can repeatedly press the SYNC button to toggle between Audio Boxes. By releasing the SYNC button on a specific signal value, the Audio Box address will display following the display of the signal for that particular Audio Box. Since the Audio Box does not show its own address, you can power on each Audio Boxes individually to test signal. For more information on signal strength please see, Signal Strength Simplified.