1) There are a few subtle differences between softtop and hardtop hoods, the most noticeable being the two footloops in front of the windshield on softtops (to tie down the windshield when folded). Also, slantback hoods had armored grilles and armor stripes around the top grille. However, nothing is impossible; my USMC M1045A2 had all that armor underneath the hood, but also the footloops. If you swap hoods, make sure to take over all the slantback parts.
2) Early slantbacks had turrets with cat´s ears on the left hand side. Later slantbacks had completely round turret tops. However, I´m not exactly sure when the change happened.
3) As far as I know, the only really sure proof (stickers can be forged) are the casting signs in the middle of the engine valley. If there are two triangles pointing awy from each other, then it is a GEP engine. Not always easy to see, as the valley is often full of crud.