How do you differentiate between lymphoma and myeloma?

Lab testing. There are distinct differences between myeloma and lymphoma which can be determined by the pathologist using light microscopy, flow cytometry and immunoflourescent staining.
Different disease. Lymphom and multiple myeloma are completely different conditions- different diseases, originated from different clonal cells, different manifestation, different labs, different in almost about everything. A good history, examination, laboratory findings, bone marrow biopsy result, imaging studies would be able to differentiate lymphoma and myeloma.