For fireplace kits, an effective method to ensure that the outer mortar between the bricks is intact, is by shining a flashlight down the chimney to look at the mortar inside. Crumbling mortar should be replaced. Metal chimneys should not include dented or rusted metal parts, and screws should not be missing.
It is a great idea to clean chimneys at frequent intervals as there may be deposits of creosote that can cause a dangerous chimney fire. Creosote is a hard and crust like substance produced during the combustion of wood. Soot also, is a flammable deposit that has to be removed if deposited on the chimney walls. If this deposit reaches a depth of