Yogurt that has gone bad will have a small amount of liquid at its surface, curdled yogurt towards the bottom of the container and mold. Normally, yogurt lasts 2 to 3 weeks in the refrigerator, and can last 1 to 2 months in a freezer, assuming it has not been opened. Once opened, it lasts about a week in the refrigerator and 1 month in the freezer.
Manufacturers are required to provide a sell-by date on their yogurt, and usually will guarantee that their yogurts are still good for up to 7 days after the sell-by date. This is why it is important to use good judgment when deciding if yogurt has actually gone bad.
Most yogurt already has liquid pooled at the surface. This liquid is usually whey, which naturally occurs in yogurt and contains nutrients. The whey should be mixed into the yogurt. When yogurt does not naturally contain whey pooled at the top, liquid is an indication that it is no longer fresh.
Live bacterial cultures in yogurt act as a preservative for the product, and help to keep it fresh for longer. Yogurt can stay in the fridge after being opened so long as it is re-covered with its foil seal.