Make a trial account in the last name of the account you scammed. Then, attempt to transfer a character from the scammed account to the trial account.
Blizzard's transfer system apparently does a last name check before it does a trial account check. So, if when you try to transfer it, it says "the destination account is ineligible for transfer", then the last name is wrong. If the last name is correct, the system will verify that but then see that the destination account is a trial account, so it'll say something like "the destination account is a trial account, you can only transfer characters between two registered accounts." Meaning, if the message you get says "you can't transfer to trial accounts", then the last name matches.
In short - when you try to transfer, if you get "the destination account is ineligible for transfer", then the last name is wrong.
If you get "you cannot transfer to trial accounts", then the last name is right.