The best way to know when potatoes are bad is to look at them, feel them or smell them.
When potatoes go bad the potatoes will begin getting wrinkly and go from being firm to soft and when the potatoes are really bad they will start smelling bad.
Fluid which is water will leak out of the bad potatoes as well and if you have one bad potato in a bunch of other potatoes in a bag you need to remove the bad potato before it makes the other potatoes bad.
If the potato has just a small bad spot on it you can cut the bad spot off the potato and still eat the rest of the potato as long as there's no worms or other bad stuff in the potato.
If the potato has sprouts it's still good to eat and you can just cut the sprouts off and eat the rest of the potato.
Sprouts on the potato does not mean it's bad though but it means the potatoes are on the way to being bad.
Potatoes usually last for up to one to 2 months depending on the temperature in the room they are stored in.
To keep potatoes from spoiling as fast make sure to keep the potatoes in a cool dark place such as a cool pantry or cool dark basement.