How Do I Know If I've Been Bugged? by Monty Henry
... You notice small pieces of ceiling tiles, or "grit" on the floor, or on the surface area of your desk. Also, you may observe a cracked, chipped, or gouged ceiling tiles, or ones that are sagging, or not properly set into the track. Prime indicator that a ceiling tile has been moved around, and that someone may have installed a hidden video camera or other eavesdropping device in your office or near your desk. Also watch for cracks or chips in the ceiling tiles.