The quick test will be a point of care test (like a glucose meter but for hb) - they'll then collect more blood to check much more detailed tests - these will give good indications of why the hb is so low - blood count is pretty helpful for leukaemia as you'd find way too many of one type of blood cell. Most common is low iron, also possible low b12. Then there's other things but basically boils down to not making enough blood, or losing too much.