Bone marrow biopsy. There is a lengthy thread on people's varying experiences with it at https://connect.mayoclinic.org/discussion/bone-biopsy-pain/

The effects of my two BMBs were minimal: https://connect.mayoclinic.org/discussion/bone-biopsy-pain/?pg=14#comment-1168936

Some centers test for cells with CD38 and CD138 antigens on cells in the blood - a liquid biopsy in a sense. This is what I have had done for the last yen years. (I was first diagnosed with MGUS in 2002.)

Plasma cell proliferation happens in bone marrow. 10-60 percent is indicative of SMM. Also plasma cells can be seen in a differential of venous blood work where they are not normally seen. Bone marrows have to be performed and flow cytometry needs to be done to evaluate cellular components in bone marrow.