If you open a bottle or thaw a bag of milk and it smells like its gone bad or tastes sour then it likely has gone bad and should be thrown out. Many times a baby will reject milk if it does taste like their regular milk (not all babies will reject bad milk).
If you are finding that a lot of your breast milk seems to be bad, you might have extra lipase in your breast milk. This causes the milk to break down faster than the average milk, hence it goes bad faster. Don't worry you can still pump and give your baby breast milk, but you should scald your milk before freezing it and pay extra close attention to the breast milk storage guidelines.