Rocket or elevator? I realize the gravitational effect in a rocket can be equivalent to the gravity on the surface of Earth. I always hear there is no way to tell the difference between the two but if there were what are the implications? 
Einstein's light elevator thought experiment
In this situation light entering the cabin on one side would appear to bend as it moves across the room to the other wall. 
For a rocket to maintain the equivalent of one G it would need to accelerate continuously going faster and faster and faster.
Eventually the rocket would be going so fast that the lights spot on the other wall would appear to be moving further and further down the wall as a rocket went faster and faster.
Standing in a cabin on the surface of the earth would not have this same moving light spot.
Another way to tell the difference is if you were standing in a cabin on the surface of the earth and you had two plumb bobs. They would both point toward the center of the earth and technically there would be a big difference between the two plumbs if you measured at the top compared to the bottom.
Could these observations make a difference in how we interpret gravity? Bending of space or deflection
 A: Re the plumb bobs, the equivalence principle tells us that gravity and acceleration are locally indistinguishable.
Since no gravitational field is uniform, an extended measurement can always tell a gravitational field from acceleration by measuring the change in the gravitational field with distance. However this is not a local measurement. The definition of local is a bit of a cheat since we usually take it to mean a distance small enough that the variation in the gravitational field cannot be detected. More formally a local measurement requires that only an infinitesimally small displacement be required to make the measurement.
Your plumb bob experiment is not a local measurement since it requires you to make the strings long enough to detect that they are not parallel.
Re the lift, the light source is attached to the lift wall so the light source is stationary with respect to the lift. That way only the acceleration contributes to the deviation of the light beam from a straight line.
