When Parmesan cheese is bad or is going bad the Parmesan Cheese will start to change color from the white color to a more yellow color.
This change in color is an indicator of the Parmesan Cheese going bad and if the color of the Parmesan Cheese changes from white to a more yellow color then it's bad and should be thrown out.
Also when Parmesan Cheese goes bad it will start having a bad smell to it along with the change in color and if you notice a bad smell from the Parmesan Cheese or it has changed colors then it's bad and should be thrown out.
Fresh Parmesan Cheese should be a white or beige color.
Sometimes mold may also be present in the bad Parmesan Cheese and if so throw it out.
Blocks of Parmesan Cheese are good for about 6 weeks once opened and if it's Parmesan Cheese that is grated for putting on Pizza and other things it can last up to a year unopened but once opened the Parmesan Cheese will last for about 6 months.