There are a few ways to know whether your mac and cheese are bad. It may smell a little off, or if you can smell the cheese from other food in the fridge. Either way, it’s best to discard the dish. Although mac and cheese can be kept for up to 3 days in the fridge, if you’ve left it out for more than a few hours, it’s a good idea to throw it out.
If it has a strong, unpleasant smell, it may be spoiled.