There are three primary things to look out for to see if jelly has gone bad – it smells bad, there’s mold on it, or it has changed color.
If your jelly smells like yeast – it’s time to throw it out. At the same time, if mold is on the jelly, bacteria have compromised it, and it is no longer suitable.
Finally, it’s probably bad if the jelly becomes brown, black, or another rotting color.