How do you know if your child has leukemia or not?

To see pediatrician. You should bring your child to see your pediatrician and dicuss further with her/him in detail about possibilities. She/he will examine your child and order the necessary initial blood work. If there is anything suspicious then she/he will be able to refer you to see a pediatric hematologist/oncologist for further evaluation/testings i.e. A bone marrow biopsy etc.
Leukemia diagnosis. Although there are various symptoms (what you feel/notice) and signs (what a physician finds) in a child iwht leukemia, a definitive diagnosis requires that a specialist examine your blood. Often a bone marrow test (performed by a hematologist or hematologist/oncolgoist) is necessary. Leukemia can progress rapidly, so seeing a physician should not be postponed; earlier diagnosis can be life-saving.