There's no way to verify the balance offline. But, generally speaking, the offline mode is only used if there's a network/connectivity failure, and there's generally no way for a fraudster to know in advance if that's going to happen.

The acquirer (ie: the merchant's bank) accepts liability in the case of fraudulent transactions. There are limits on offline transaction values, and merchants that have high fraud risk may not allow it at all.

It certainly happens, very occasionally, here in the UK. You try to pay something with contactless but there's a long processing delay. The terminal will then time out and ask you to insert the card and enter PIN, and you will get something strange printed on the receipt like "CVM: offline PIN". You may then see the transaction pop up in your notifications a few hours later, or early the next morning.

Also, some merchants do offline or semi-offline transactions as a matter of course. London Buses, for example, accept contactless payments via bank card. But the transaction must be processed very quickly and the mobile data connection is not entirely reliable - the bus could be in a tunnel or a coverage black spot. So what really happens is the transactions are batched and processed at the end of the day (this also allows them to do some post-processing for multiple-journey discounts, daily price caps, etc).

Someone like London Buses isn't overly concerned about fraud because the transaction values are low. But they do have a way to blacklist cards numbers that are known to be bad.

There are only half million Teslas so you can just distribute the VIN numbers of all of them with a single bit flag saying whether they are eligible for free charging and another bit saying that they have an account in good order. Twenty bytes per car should do it, say 10 MB of storage to hold enough information to say which cars are allowed to charge even if the network is down.

Mind you in three years of owning and driving (70k km) my Model S I have never encountered a Supercharger station that was entirely out of order, just a few heavily used ones that have one or two stalls often not working.

Send invoices to identifiable users, which go to collections if unpaid.

Eat the cost for unidentifiable users. As long as you can fix any connection problem within a few days, the maximum value lost is limited by physics.

People used to pay for stuff with checks all the time. It's similar to that.