The tube muscles are on the bottom...it's what the fat cap covers. There are 4 decent size muscles. One is the largest at about 3/4" diameter and a couple at about 1/2" diameter.
If you are doing what DawgPhan suggests, you need to keep doing it. And pay attention. You will soon learn how the butt is made up of the different muscles and how they run.
No two are alike...because you are getting two different pigs' shoulders in a twin pack. But they are all similar.
If you are having trouble separating the muscles, you aren't cooking it long enough.