The way to tell if homemade yogurt is sour is by looking at the texture. If the yogurt is soft and wrinkly, it’s most likely spoiled. You can also look for mold – green or white – on the top surface of the yogurt. While it’s not harmful to eat a spoiled jar, a moldy container means it’s contaminated. The smell is the most reliable way to tell if homemade yogurt is terrible. A sour or rancid smell can indicate a spoiled product.