You can tell when butter has gone bad by smelling it and looking at it.
When butter goes bad the butter will smell rancid and can have mold growth on it and also may change color or become hard.
If the butter has mold, smells rancid or looks bad then throw it out and don't risk getting sick.
Butter can eventually go bad in the fridge.
Butter will keep in the fridge for for up to 3 months as long as you keep the butter covered in a sealed container.
When butter is left out at room temperature it can go bad within a few days so always keep the butter refrigerated when not using it.
If the butter smells rancid has mold or has changed color then it's bad and should be thrown out.
When butter goes bad the spoiled butter will smell rancid.
If you smell the butter and it smells rancid and makes you want to vomit then throw it out because it's bad.
The spoiled butter may also have mold or the color or texture of the butter can change and if so throw it out.
Butter should last for a few days at room temperature and when refrigerated butter can last from 1 to 3 months.
When leaving butter out at room temperature to soften it's usually safe to leave your butter out for up to 2 days.
If you leave your butter out longer than a few days then you risk it getting spoiled and growing bacteria.