When mashed potatoes start getting too old, the liquid begins to separate from the solid. Initially, the liquid is almost transparent, but in time it turns milky, and the mash will develop a sour smell. If this happens - dispose of it. As well as smelling sour, mash that has turned will also taste sour. The other clear sign that mash has turned is when spots or patches of mold appear. When the mash is left too long, it begins to ferment. It may not be noticeable to the naked eye, but your nose will surely detect it.