Leukemia typically shows up in routine bloodwork--the complete blood count
(CBC)--as an elevated white blood
or an extremely low white blood
. Sometimes, it also presents with a low red blood cell count. Your father's doctor may have mentioned that he exhibited signs of leukemia either because of the symptoms you mentioned your father having (fatigue, weakness, bone pain, etc.), or because your father had an elevated white blood cell count with a CBC the doctor had run.