When canned clams are bad they will usually have some mold growth on them or they may have a bad smell to them.
If you notice a bad smell on the canned clams or there is mold growing on the canned clams then they're bad and should be thrown out.
Or if the canned clams have a bad or off taste to them then they should be discarded because they could make you sick.
Also if the can that the canned hams are in is leaking, rusting, bulging or severely dented you should discard the canned clams because they're likely bad as well.