How indicative of leukemia is pain in the arm?

Not likely: There are many reasons to have arm pain and leukemia is fairly low on the list. There can be bone pain associated with leukemia in general.

Non-indicative: Pain, is probably the most common symptom for anyone who presents to a doctor. Pain is a symptom. There are likely 1000 different conditions which cause pain. Leukemia, or cancer of the white blood cells, can be ruled-in or ruled-out, starting with a simple blood test called a cbc, or complete blood count. I would discuss with your doctor your concerns and go from there. Good luck.