Ask to see a purchase receipt along with the original credit card. This is a good way to see if the scalper actually bought the tickets. But it is unlikely even the most honest scalper will be willing to verify a credit-card number in a virtually anonymous credit-card transaction. This step can be applied to physical or e-tickets.
Insist on getting the ticket's serial number. All event tickets that are not sold at the ticket window on the day of the event come with a bar code and serial number. You can use this to verify the validity of the ticket with the issuing company, such as Ticketmaster. This will also help you as a follow-up step in avoiding the other fake ticket scam: the ticket that was real, but has been reported lost or stolen in order to secure a refund. This step can be applied to physical or e-tickets.
Make a physical comparison between the suspect ticket and a recent, valid ticket from the same vendor. This is unlikely to catch a sophisticated forger, but many forgeries are actually not that sophisticated and can be caught with a through examination under a magnifying glass. Also check the paper quality and texture. This step can only be applied to physical tickets.