Spoiled milk is either past its expiration date or was handled improperly. You know your milk is no longer drinkable when it smells sour and tastes off -- with a flavor that's stale or outright bad. Curdling and thickening can also occur when milk spoils. Pay close attention to the sell-by dates when you purchase milk in the store and store it properly once you get it home.
Bacteria grows more readily in warmer environments, so maintain cool temperatures -- 40 degrees Fahrenheit or below -- in your refrigerator. The back of your refrigerator, away from the light, tends to be cooler than other spots.
Avoid detours on the way home if you've purchased milk. The sooner you can get it back into a refrigerated environment the better. Milk should remain fresh for one to five days longer than the sell-by date on the jug or carton. If it spoils earlier, chances are it was mishandled and not refrigerated properly.