How can you twll the difference between sciatica and bone cancer?

I've had sciatica pain in my left leg over a year now. It got better when I was seeing my chiropractor 3 times a week, but now that I'm only seeing him once a week, it's gotten bad again. It is very painful and difficult to walk sometimes. It is interrupting my sleep because now it's even painful to sleep on my left side. I also have pain in my left arm. How do you tell the difference between sciatica and bone cancer?