If I remember right, it doesn't always show up on an MRI. I had optic neuritis in 2006 and it didn't show up on the MRI. I had tiny "nonspecific white dots" in some other spot that wasn't anywhere near the optic nerve. I've had optic neuritis a couple of times since then, but I pretty much refuse to get back into another MRI machine because I have claustrophobia. (didn't know I was that claustrophobic until that first MRI in 2006)

They might be able to tell from looking at your optic nerve thru an ophthalmoscope. But, again, it might not show up. The color of your optic nerve can change when it's inflamed. (think it gets a pale pink color?)

I'm pretty sure that you can have a flaming case of optic neuritis and have it not show up in an MRI or looking thru the ophthalmoscope. There's other signs to look for.