The dal may be spoilt if it has a sour smell, is slimy, or has black spots on it. If the dal has any of these characteristics, do not eat it. It is best to throw it away to avoid any further issues. Generally, dal turns bad when you store it outside at room temperature. So, it is important to store it in a cool, dry place. You can also refrigerate it. If you are not going to use the dal for a few days, freeze it. This will help to extend the shelf life of the dal.