If its someone local to you may possibly be able to check by placing in your craft and open the dji go 4 app and check battery status, not sure how much information you can gain via the app, I personally would not concider purchasing a used flight battery because if the battery has been abused ie, run down to lower voltage or left fully charged for long periods all shorten the battery service life, also you would need to know how many times the battery has been charged and used and is the seller if asked going to tell you the truth !
Its to late when your craft drops unexpected out of the sky due to a battery fauly..
Offer them a low low low price. They aren't worth much as an unknown quantity and where else is he going to sell them? Remind the seller that (a) they are a single, potentially catastrophic point of failure, (b) that you can't verify their condition or previous use, so (c) you're taking all the risk. Once you get them you can check them out properly - if they turn out rubbish, well you didn't lose too much did you.