YES, there is an absolutely foolproof way to listen to a particular encrypted channel.....
First, quit your current job.....
Now that you have a little extra time, start a daily work-out regime to build up your strength and stamina to a point where you will be able to pass the police physical exams and survive the Academy PT training for the 12 to 16 weeks. Be sure to go to the library daily also to check out some books on criminal and evidence law, physical crime scene preservation, sociology, psychology, leadership techniques, unarmed defense and martial arts, You may also want to enroll at a gun range for training with non-lethal and deadly-force weapons. While you are doing this you may want to start practicing some written communications skills, because as the saying goes; "if it isn't written down, it didn't happen." Then there is the math/physics of objects in collision you'll need for accident investigation. While you are doing this, you may also want to write to all of your old employers to be sure you have good addresses on them and that they are willing to give you a good recommendation when the department starts your back ground check. Depending on what department you are wanting to listen to, you may need to go back as far as high school teachers because the department will do so. Oh, I almost forgot. There is also the driving skills you'll need. Gee, I hope you haven't had too many tickets over the last 10 years or so. That would make all the above a waste of time.
OK, now that you have prepared your self properly, get a job on the department with the encrypted communications you want to listen to that gets you into a position that causes them to issue you a radio with the correct de-scrambler equipment.....
Otherwise Title 18, USC Chap 119-2511 says you are just out of luck if you are in the U.S.