With pork shoulder or butt, the marbling is important for a long cook where you are essentially slowly rendering all the fat out throughout the cook, in order to get moisture into the meat. Look for fresh pink color with no odor. The fat cap and the meat should be firm. Buy bone-in shoulder. The bone helps keep the uniform squared shape for a shoulder and allows for more even cooking. The bone also acts as a temperature gauge for when the pork is done when it can easily slide out from the shoulder. However if all you can find is boneless, that is fine as this recipe works the same!