How can an eye oncologist tell is there is a malignancy behind the eye without a biopsy?

Depends: If you mean behind the eyeball, that is something you can only see on an MRI and often times it is difficult to do a biopsy. A neurosurgeon will have to do it. If behind the eye you mean on the retina, then cancers in this area usually have a typical appearance (like melanoma or metastasis or lymphoma) and you can make the diagnosis without biopsy.