Is It the Battery or the Charger?

Hi - novice battery powered tools user here. I have 2 Ryobi 18v power tool batteries that the Ryobi charger says are fully charged. Neither of the batteries will power a Ryobi drill or a reciprocating saw. I am about to go get a volt meter to check the charge in the batteries. These are old batteries - I'm thinking 20 years old! - and they haven't been used in years and have been stored in both hot and cold conditions, so I'm thinking they're shot but how do I know for sure? If they aren't holding a charge, why would the charger say they are fully charged? If they are holding a charge...what is the next step in troubleshooting? Thanks for your help.