Signs to look for in a rotten sweet potato:

• Soft or mushy texture. A good sweet potato should be firm to the touch. If it's soft or mushy, it's likely spoiled.
• Discoloration. Look for any dark spots or patches on the skin. These could be a sign of rot. If the flesh inside the potato is not a vibrant yellow or orange, it's also best to discard it.
• Mold. If you see any mold on the sweet potato, don't eat it. 
• Smell. A fresh sweet potato should have a mild, earthy smell. If it has a sour or off smell, it's probably gone bad.
• Sprouts or “eyes”. This means you are likely looking at a pretty old sweet potato. While it’s possible to scrape off small ones, “eyes” that are more than half a centimeter long means the sweet potato may not taste good or have other unseen signs of spoilage.
• Any combination of these signs means it’s best to discard the sweet potato and not eat it.