I have heard of eye doctors using a MRI, CAT, VEP/VER (to check the speed of nerve transmissions down the optic nerve), and opthalmoscope on the back of the eye (so long as the inflammation is not further back along the optic nerve), to test for ON.
However, I would think ON would be most obvious, by any testing method, while it is inflammed and we are experiencing symptoms. I'm not sure how much damage they can pick up on if it is not active.
They can probably give an educated guess as to whether what you experienced previously was ON, but I'm not convinced they could confirm that at this point in time though.
Cherie