The difference starts with the type of cheese. Like mushrooms, you really don't know a bad one unless you've educated yourself on its characteristics. However, a cheese gone bad isn't necessarily going to kill you. The main culprit behind the cheese stink is the washed rind category. Washed rind cheeses are repeatedly bathed in a brine during the aging process. Some brines include wine, beer or spirits, adding an extra dimension of flavor to the cheese. While the washing protects the inside paste and helps develop a strong, meaty flavor, it cultivates the growth of brevibacterium linens, which is the source of what I call the "rotting feet" smell. Lo and behold, brevibacterium linens is also the cause of human foot odor. Think of raw meat that has sat in the sun too long - it's stinky, but not quite the stink of decomposition. Tallegio, Pont L'Eveque, Limburger, Red Hawk, and real Muenster (not that American crap with the orange coloring) are all stinky but deliciously buttery.
Other cheeses have a moldy fish smell. The Basque Ossau Iraty or Italian Fontina Val D'Aosta both come to mind - their rinds smell like dirt and shrimp shells but they taste nutty, meaty and slightly sweet. Blues like Roquefort and St. Agur smell earthy and sour. Aged goudas smell of butterscotch and beef, sometimes with an after-scent of red wine. Bloomy white mold bries and triple creams smell of earth, mushrooms and sweet butter. Like I said, to know the right smell is to know the cheese that's emitting it.
So what is bad bad smell? Ammonia. Really - that's just it. The chemical odor that burns the nose and makes your face scrunch. When you smell ammonia, your cheese isn't aged - it's rotten.
