Hi L.!
Does the baby naturally roll over to her belly while sleeping? My first daughter always slept best on her belly as a tiny infant, but I only allowed it when she was napping adn I was up and about to watch over her. When my triplets were in the NICU, they actually put the babies on their bellies a few times a day for a nap. etc.... they said it was good for digestion. When I would put my babies down to sleep for the night, I put them in between the wedges/sleep positioner on their backs, of course. When they started rolling over naturally and I'd wake up to find them on their bellies, it was time to take out the positioners because obviously they were no longer serving the purpose. With rolling over naturally, I noticed they knew what to do with their heads... turn them to the side instead of lying face down. Maybe it all comes as a natual process.
If you are nervous about this, there is a baby monitor called the Angel Monitor that you can get that may give you piece of mind. It's kinda pricey ($100 for the unit -- they have it at Babies R Us), but it acts as a reg. monitor and is also a movement monitor... should no movement be sensed within 20 seconds (movement as slight as a breath), an alarm goes off to alert you. I used it for my babies... always gave me a piece of mind. Just a thought...