To tell if an avocado is bad, you must cut it open and feel the inside. It should be smooth, creamy, and slightly bumpy. If the skin is black or has a slimy, white coating, the avocado is probably rotten. It also needs to smell pleasant but not musky. If it smells like rotten eggs, throw it out. If it has spots or discoloration, you need to throw it out.
To You should determine the ripeness of an avocado, cut it in half and squeeze the center.