Know what I hate about answering a question like this? How do I (we) know, you aren't asking this so you can clone one???
OK, look at the aluminum Shelby Vin tag under the hood. Does it look brand new, but the car looks old? Or does it look to be as old as the car. Yeah, some folks have put on new plates, but...
ALL 67 Shelbys had this same number stamped into the top of the right side fender mounting flange, near the shock tower.
And, this is difficult, unbolt the right fender and pull it back to reveal the Ford vin. First thing is the plant code better be for the San Jose assembly plant. If not, well it's a clone.
Contact the SAAC registrar with both the Shelby vin, and the Ford Vin. SAAC will tell you if they match their records. SAAC WILL NOT RELEASE TO YOU THE FORD VIN.
Does it have the complete deluxe interior? rear fold down seats, front power disk brakes? roll bar with the Y shoulder belts?
Other things, but any of these could be changed due to wear, unavailablity, etc: 140MPH speed (not 120), factory tach, the real steering wheel, Ford 9" rear end, larger (wider) rear brake drums, the scoops, don't forget the real Shelby has a longer front end, so, it will not have a stock Mustang hood, and my favorite: look for the rubber/cast steel rear suspension rebound thingies toward the front of the rear springs, bolted to the bottom of the floor above the rear springs.
And of course, look undernearth the front fenders for welding/filler in the area under the vin.