Inspect the outer leaves of the cabbage for any discolouration, wilting, or spots. If the leaves are slimy, mushy, or have a foul smell, then the cabbage has gone bad and should be discarded.  You can also cut into the cabbage to check for any signs of spoilage, such as brown spots or an unpleasant odour. If the cabbage looks and smells fine, you can still give it a taste test to make sure it's still fresh and crisp.  For cut cabbage, remove any darkened parts and check for other spoilage signs.