Cabbage doesn't require much in the way of spraying to grow it, so it doesn't contain as many pesticides.
Look for cabbage heads with tight leaves (the exception is savoy; its leaves are loose and crinkly naturally), and the head is heavy and firm. The outer leaves should be shiny and crisp and avoid leaves that show signs of yellowing.
Discard the outer leaves before use and store in your refrigerator crisper.