This +1. That is the fastest way to pick a GP60 out from any other geep. Of course there is the internal difference that GP50's used 645's and GP60's used 12-710's. As far as ES44AC's and DC's, well its the numbers, and the fact the DC is a DC traction motor and an ES44AC uses an AC Traction motor.The most noticeable difference between the GP50 and GP 60 is the dynamic brake blisters. The GP60's dynamic brake blisters have a more square look to them. That was the only locomotive I know of that used them. There is other smaller differences if you know were to look but that is the easiest to spot.
Bill