How do you tell the difference between a pineal tumor and a pineal cyst?

A biopsy or imaging: True pineal gland cysts are not brain tumors; rather, they are a completely benign cysts. The large majority of pineal gland cysts do not enlarge much if at all. However, some do enlarge over time slowly. Even though pineal cysts are benign and usually harmless, they can mimic a tumor, they often need to be distinguished from other cystic tumors via biopsies or imaging study (ie MRI of the brain).

Imaging studies: I believe MRI is the best imaging modality for viewing the structure of parts of the brain. An MRI should be able to visualize the pineal gland and determine whether it is solid, cystic or enlarged.