Can leukemia be detected through a simple blood test?

Maybe. At times a blood test can reveal leukemia, more often, it reveals a suggestion of leukemia (decreased blood counts) that will require additional tests. The best test to know about leukemia when there are these suggestions, is to have the bone marrow examined. This will definitely show leukemia, if it is present.
Generally. Fully developed leukemia can be detected through a blood cell count and examination of peripheral smear. Many patients require bone marrow examination and special studies to fully classify the type of leukemia. Treatment depends on the detailed classification.