The FCC publishes the frequency assignments for aeronautical radio usage (Part 87). You may wish to "listen" anywhere, but technically one should be monitoring guard when a receiver is available.
Addendum #1 Protected frequencies, such as around guard, are implemented as "channels" which are effectively wider. From a practical standpoint, this allows "weaker" signals to be heard without splatter or bleed over from adjacent channels. In the case of guard, a time honored technique is to tune a radio off channel, and use that as a metric of how strong a signal is. This is helpful if flying a grid, looking for an ELT, as an example. I have never been in CAP, but I have located several downed aircraft, and participated in one manner or another of helping locate perhaps dozens of others over the years. If you want to practice the technique, you can use some continuous transmitter, such as AWOS. Newer radios have better off channel rejection than some of the radios which have been retired in recent years because they did not accommodate current channelization. So this technique may not yield the same information about the strength that an oder radio might. Nonetheless, knowledge of this method may prove helpful at some time.
And about all those downed aircraft? A large number of them were not crashes but rather planes tied down, and where no one apparently listened on 121.5 after that last hard landing. Higher flying aircraft would pick up an ELT, and advise center or in ancient times, FSS. They in turn would request aircraft in the vicinity to report whether they heard an ELT, and create a most probable map of where the ELT was. Most of the time, would find ELTs at night sitting on the ground at an unlit grass field, with a mercury vapor light nearby, lighting the aircraft.
Addendum #2: To answer the OP question more directly, tune to 121.475. If you hear anything there, you might go to 121.5 because there is likely a strong signal on 121.5. It could be a downed aircraft in your vicinity, or it could be a plane intercepting you, or something else possibly worthy of your attention. But this adjacent channel should be quiet most of the time, and should satisfy your need for a quiet frequency.