No one has been able to answer?? THis is a very good question that I also have.

If a doctor notices your routine blood tests are not normal they can test your blood further, there are specific blood tests that are done to detect certain cancers, they look for "markers" in the blood. It all depends on what type of cancer the doctor is looking for such as leukemia, as for cancer of organs usually scans will show cancer and the blood work will be out of whack.
They then put these together and come up with an answer to why the blood work is not normal and what came up on the scan.
I am trying to get answers myself and have been doing a little research. Sorry if this doesn't answer your question fully as I am learning about this myself.