Some cancers produce tumor markers which can be detected through blood work, but the physician would have to suspect the particular type of cancer in order to justify ordering the specific test. Specific tumor markers include CA 125 (cancer antigen for ovarian cancer), AFP (alpha fetal protein for liver cancer), and PSA (prostate specific antigen for prostate cancer).
Blood protein testing can also be observed by electrophoresis which can detect abnormal immune system proteins; a marker for multiple myeloma. Getting blood work more often provides the physician with clues that may allude to a potentially cancerous state, from decreased white blood cell count to a decreased red blood cell count.
To be absolutely sure when making a diagnosis, more invasive testing is often needed, such as a biopsy.