Normal signs of age
A hard cheese can grow white crystals. Those crystals are not mold. A washed rind can smell of ammonia when it is ripe. A little of that smell is normal. A harsh ammonia smell means the cheese is past the point where it tastes good.
When to trim
A firm or hard cheese can grow a spot of mold on a cut face. Cut that spot away with a margin of cheese, then use the rest. Do not let the knife carry mold back onto the clean face. A dry edge can be trimmed the same way.
When to throw it out
Throw out a fresh cheese or a soft cheese if it grows mold. Throw it out if the paste is slimy, or pink when it should be white. A bloomy rind that has collapsed into a bitter liquid is past eating.