Dear seven7: A bone scan is designed to show damage to bones. It usually differentiates between the two based on patterns of damage. For example, arthritis tends to be located near joints and symmetrical. Bone metastasis, tends to appear more randomly. This is a bit oversimplified. If you are concerned, ask the doctor or radiologist to show you the scan and explain the differences.
Usually. The changes seen in arthritis are usually right at the joints; with cancer the changes are typically in the long parts of the bones.