Use Ticketmaster's website to look up the seat numbers on the tickets to see whether or not they've been purchased. If they have, you are one step closer to proving that the tickets are legitimate.
Check an event website or schedule to see if the time and date on the tickets corresponds to the actual show. It's important to pay close attention to these details.
Trace the history of the ticket by furnishing receipts. If the tickets are from Ticketmaster, the receipt and confirmation will say so. If no history of the tickets can be furnished, this might be a sign that the tickets were stolen, counterfeited or previously used.