When mustard goes bad an easy way to tell if the mustard has gone bad is to look at it and see if it has any mold in the mustard or if the mustard has changed texture.
When mustard is starting to go bad it dries out and separates overtime but even then the mustard is likely still okay to eat and you can stir it to get the texture back.
But if it has mold in the mustard or it smells bad or sour then throw it out.
The mustard will most likely have a sour smell to it that will make you want to gag or vomit and if so throw the bad mustard out and don't risk your health.
Mustard should keep in the refrigerator for up to a month or two and if the mustard hasn't been opened it should last for 6 months or longer.