Open Question: How can I tell if My doctor gave me a Placebo?

I have been on January 2010 and I recently changed doctors. I asked my new doctor for an Ambien Cr RX and he was a bit hesitant, but he prescribed it anyways. After using it for a few days I still have not been able to fall asleep, I don't even become drowsy. My question is, how can I tell if my Dr gave me a placebo?