When apple cider vinegar goes bad the liquid of the apple cider vinegar usually becomes cloudy and there's rust-colored sediment at the bottom of the bottle.
When you open the bottle of apple cider vinegar and it's bad the apple cider vinegar will also have a bad or pungent smell to it.
If the bottle of apple cider vinegar remains sealed it will usually last for up to 2 years before it loses it's freshness but as long as the apple cider vinegar remains sealed it will remain safe to consume for up to about 6 years.
Once opened the apple cider vinegar will usually last for 6 months in the refrigerator.
If it smells bad or tastes bad or has any mold or other bad looking colors in the apple cider vinegar then throw it out to avoid getting sick.