Actually, it's not too hard. I put a GT take-off exhaust on my 94 V6, and a lot of folks mistake it for a GT by sound alone. I even fooled a friend of mine, who's heard the car before. He heard a Mustang driving up, and thought it was a GT. To his great surprise, I pull up in the V6.
Like super302 said, check the taillights and the rest of the car. I believe the 94-95 Cobra's has the "crab" spoiler with the 3rd brake light in the spoiler. That's a dead giveaway if the 'stang doesn't have that one, or has two 3rd brake lights.