If the pickles are going bad they'll usually have a bad smell to them or start to shrivel up when they go bad.
If the pickles have a bad odor to them then they're likely bad but if they smell good then the pickles are still likely okay to eat.
As long as you keep the jar of pickles properly sealed up and in the refrigerator they should easily last for a few months past the use by date.
If the pickle jar has never been opened then pickles can last as long as 2 years safely past the use by or eat by date.
If the pickles don't taste good then they're probably bad but it can be hard to tell if pickles have gone bad other than by looking at the pickles or tasting or smelling the pickles.