I would take that to court. All you need to do is create reasonable doubt, and if it "looked like" you were going 75, but actually weren't, thats a B.S. charge.
Did he write on the ticket that it was just what it looked like? I would ask for full disclosure on the ticket then take it to court.
There is such a thing as moving radar, so if he had used that and got you at 75 I would think it would be harder to beat, but without a radar reading... I would ask for it to be thrown out.
As for a smartphone app, I would be surprised if there wasn't, I have a speedometer app on mine that is more accurate than the speedometer in my car, so I'm sure there has to be one out there that can track it. A company I used to work at had gps tracking on the vans that could tell the boss where the vehicle was and how fast it was going, and it even had speedlimits built in so it could email the boss alerts when you were speeding!