There are several ways to tell if apple cider has gone bad. The first thing to look for is any signs of mold, which can appear as a slimy or fuzzy coating on the surface of the cider. Secondly, you should check for any off-putting smells, as cider that has gone bad will usually have a sour or acrid odor. Finally, you should taste the cider to check for any off-flavors, as bad cider will typically have a sour or bitter taste. If you notice any of these signs, it is best to discard the cider as it is no longer safe to consume.