The quickest way to tell is by the chimney outside. A chase (for a pre-fab fireplace) is wood-constructed and has a decorative surrounding typically covered with vinyl, aluminum or wood siding, or sometimes simulated stone.Pre-fab fireplaces are installed on site from UL listed components made in a factory. A pre-fab chimney has a multi-wall insulated metal flue to vent smoke out of house, with a metal cap to prevent water and debris intrusion. A pre-fab fireplace can be removed if damaged and replaced with a new unit for less cost than masonry.A masonry / brick fireplace and chimney is constructed on site by brick masons using brick and terra cotta tile for the liner, and is more expensive to build and maintain. Many masonry chimneys do not have chimney caps installed, allowing water and animals to get inside the structure and sometimes inside your house.Pre-fab fireplaces are not any more or less safe than masonry fireplaces.