Great question! Sight, smell, then taste - these are your best guidelines as to when to know if a cheese has passed beyond the point of no return.
If the expiration date printed on the cheese packaging has come and gone use the following tests to determine if your cheese is still usable, or if it needs to go to the trash.
Most whole, cut or sliced cheeses can survive a little mold and still come back from the brink. The more moist the cheese, the more likely it is to succumb to decay versus the drier cheeses which are more resistant. Mold needs moisture to grow, you know.
are riskier, again depending on the original moisture content of the
cheese. Cheeses such as cheddar and mozzarella should either be bought
whole and grated as you need, or if bought pre-shredded used as soon as
Pre-grated or shaved Parmesan, however (and no, not just the green-can kind) can go much longer without risk because the aged cheese was so dry to begin with.
Cheeses which always show mold, both before and after
their "Package Expiration Date" will be Bloomy Rind cheeses like Brie
and Camembert. These cheeses have a velvety white
covering on the rind, which is edible (and delicious!). This is fine and expected.
Orange, rust red, blue, or green molds are not fine. These will be spots on or in the cheese, not over the entire cheese itself -- well, that depends upon how long the cheese has been neglected, of course -- and will be distinctive against the creamy white of the rind or the paste.
Blue cheeses, obviously,
have beneficial mold throughout which is the blue-green veining. This is
expected. Again, orange or reddish spots, or blue/green that is centered around a definite spot (not shot-through the cheese like veins) tell you that the cheese has picked up an unwelcome visitor.
Hard cheeses can and will develop some mold over time (especially if not properly wrapped and stored), and the above colors are the ones to look for.
Most of these past-prime cheeses are salvageable. Simply
cut off the moldy part plus an extra 1/8- to 1/4- inch and continue to use the
cheese.This guideline is for whole or cut cheeses, not grated cheese.
Grated cheese should be examined undisturbed. If there is a bit of mold (any color) in one section of the container, carefully scoop it out with a spoon and discard. Then examine the rest of the cheese using the second and third steps below. If you have accidentally stirred or shaken the cheese before noticing a problem, the best idea is to discard all of the cheese. It is unpleasant to toss good cheese out with the bad, but much safer in the end.
Fresh cheeses and washed rind cheeses are a different story. If you see slimy mold, or pinkish, reddish ugly patches on your fresh cheeses (like Mozzarella) or on your double or triple creme cheeses, just throw them out. They are done. Similarly with Soft- and Washed-Rind cheeses like Livarot, if the rind looks questionable, slimy, weepy, and the smell is really awful, toss it without hesitation.
How does the cheese smell? OK? Or does it have what is called an ammoniated smell?
If the smell of livestock urine is strong (often nicely called "barnyard"), sharp or bitter, throw it out. Keep in mind that
strongly flavored cheeses will always have a strong smell, and as they
age/ripen and live in your fridge, their aroma will intensify. That
does not mean the cheese has gone bad. So long as you don't gag from a sharp
ammonia aroma, the cheese is probably fine. Proceed on to the final test.
If the appearance and the smell are acceptable, but you are still not sure, take a small bite. Cheese continues to ripen after you purchase it. Its taste will therefore change while it lives in your fridge and its texture will change as well.
Goat cheese, for example, may be quite soft when purchased, but wrapped and stored too long, the texture will become much firmer and the taste will become stronger. So long as you still like the taste, the cheese is fine. Just be aware that the flavor will intensify on most cheeses over time.
If the small bite of cheese makes your tongue, lips or cheek tingle or burn, the cheese is bad (even if it passed the look and smell tests). Spit it out (do not swallow it) and rinse your mouth out with water. Toss the cheese and move on!
Remember, there is always more good cheese in the world. Don't risk making yourself sick and lose any time you could spend enjoying good cheese, just to try saving some bad.
Expiration dates on cheese are not carved in stone either, however, and just because your cheese is past its expiration date does not mean it has expired! Use your common sense and your instinct when making the decision to keep or toss a cheese that has exceeded its use-by date.
Bottom line, be sure to wrap your cheeses properly and store them properly. Cheese is a living, breathing organism.
See Storing Cheese for pointers.