The doors should always be fully open or fully closed. Glass doors should be fully open when starting a fire and when the fire is burning strongly. The glass doors should be closed as the fire dies down to minimize the amount of room air going up the fireplace chimney. When burning gas logs in a wood-burning fireplace, the glass doors should always be fully open when the gas logs are burning. ALWAYS keep the screen mesh on your fireplace closed when using the fireplace to guard against sparks

Turn on the fireplace. If the flames reach above the gas logs and the look like a wood burning fireplace, then the logs must be vented. If the flames are small, an have a blue cast, the fireplace is vent-free.