Proudly American
Locate the moisture accurately
Describe whether the moisture is on room windows, a nearby wall or the fireplace's own glass front. These are different observations. Note when occupants first saw it and whether cooling, cooking or other sources of indoor moisture were also present. Do not operate the fireplace again simply to reproduce the effect or draw a conclusion from the shape of a fogged patch.
Confirm how the unit was designed
The Building America Solution Center discusses moisture and indoor-air concerns associated with unvented combustion. A vented log set may also sit in a masonry fireplace, but its venting arrangement is fundamentally different. Ask an appropriate professional to identify the actual appliance, its instructions, the space in which it is installed and the conditions under which it may legally and safely be used.
Give a returning guest one clear status
If the equipment type or suitability is uncertain, leave it unused until evaluated. Do not treat an open window, a dehumidifier or a silent CO alarm as authorization to use it. If an alarm sounds or anyone has symptoms suggesting exposure, get outside to fresh air and contact emergency services. Keep moisture observations with the room file and any service findings with the appliance file.