The symptoms can be mild at first and are often difficult to distinguish from minor illnesses. Leukemia affects the three cell lines of the blood (red blood cells, white blood cells, and platelets), so the symptoms are related to those three cell lines. A lack of red blood cells causes anemia, meaning the patient may have unexplained fatigue and be pale. White blood cell count may be increased, decreased, or normal but, regardless, the white blood cells don't function normally. This leads to infection, and the patient may have frequent colds or ear infections, with or without fever. Low platelet count leads to difficulty in clotting, meaning the patient may have nosebleeds, easy bruising, and it may take longer for bleeding from cuts to stop.
Any or all of these symptoms may be present with leukemia, along with bone pain, but the only way to definitively diagnose the disease is with a bone marrow biopsy. However, no doctor would suspect leukemia in a patient with a perfectly normal CBC (complete blood count), a test that is common and very easily done. So the first step would be to have the CBC done.
I'm a pediatric cancer pharmacist.