Can you tell what a mass behind the retina is without removing the eye.

Yes, it Depends: Depending on the accessibility of the lesion a biopsy may taken and examined by a pathologist to arrive at a diagnosis.

Retro-orbital mass: A mass behind the globe (eyeball) is a short list of possibilities best managed by a specialist from ENT, ophthalmology, or neurosurgery who specializes in that area, called the 'skull base.' Fortunately, the diagnosis is often known or suspected before surgery (if needed) since biopsy is difficult to impossible before removal. CT and MRI read by neuro-radiologist usually gives accurate diagnosis.

Yes: Most masses in the eye can be diagnosed through testing in a retinologist office.