Ask the seller of the e-ticket to provide you with a copy of the confirmation he received when he purchased the ticket. You can use the confirmation information to check on the ticket's legitimacy.
Contact the venue or ticket seller by phone with the e-ticket handy. Provide the e-ticket number and ask the company to confirm the sale of the ticket.
Visit the box office of the venue or the ticketing company and ask a company representative to examine your e-ticket document. The representative should tell you whether the e-ticket number or confirmation number is legitimate or not.
Understand that receiving confirmation of the legitimacy of your e-ticket number or the associated order number doesn't guarantee your admission to an event. A scam artist may purchase a legitimate e-ticket and then sell copies of it to multiple buyers. You may not discover a problem with your ticket until you try to enter the event and learn that someone used the same ticket number before you arrived.