Unforunately, there isn't any way to tell. I am a Hawks season ticket holder, and I could sell someone the tickets I receive in the mail, and they would still not get someone into the stadium if I have printed a copy of the ticket from my account. The ONLY way I would ever buy from someone I don't know is if they qualify with these things.
1) Is a season ticket holder
2) Can meet me, and transfer the tickets from their season ticket holder account to my email, where I then accept them.
Once you've accepted, the previous owner loses ALL ability to copy the tickets again. You now have valid tickets sitting in your email account, just needing to be printed. If this guy seems legit, meet him at the stadium and walk up to the gate with him.