Don't waste food by throwing it out just because the "Sell By" date has expired. That's just a conspiracy by the Military-Nutritional Complex to get you to buy more food.
The following tests can tell you whether the food is still good to eat.
Meat: You open the fridge door. You hear voices shouting "Turn off the light! You're ruining the party!" Time to throw out the meat.
Milk: When milk has been colonized by bacteria, use the following test. Make a cup of tea. Pour the milk into the tea. If the colonists don't throw the tea over the side, it's still good to drink.
Cheese: If it's not moldy, it's good. If it is moldy, just cut the moldy parts off. The rest is still good. If it was moldy when you bought it, it's good forever. If it's been so long that you can't remember whether it was moldy when you bought it... then you should probably throw it out.
Sour Cream: There is no known test for sour cream. Really, the stuff comes that way. If it doesn't kill you, it's good.
Broccoli rabe, arugula, fennel bulbs: If you're reading this advice and you find any of these things in the fridge, you got drunk last night and woke up in somebody else's house.