Haunted mirrors are to be avoided when you visit an antiques store (unless, of course, chancing upon such objects is your very purpose).  When consulted to appraise a mirror's hauntedness, the first thing we look for is imperfections in the glass or silvering.  Notice in the unretouched photographic illustrations how a peculiar mirror has a deformity that warps the face of any Narcissus who approaches it.  A thing of horror — and beyond question profoundly haunted.  A more subtle issue we look for is a discrepancy between what is reflected and what is actually in the room.  This requires very careful looking at details within both worlds, and it can be helpful to take photographs of the mirror world from as many angles as possible so as to study them at leisure.  We then look at the back of the mirror for a manufacturer's mark (for example, "Pairpoint Mfg. Co., Quadruple Plate, June 28, 1904"), and if such a mark is upside down, the mirror has been hung "inverted," making it more susceptible to negative powers.  Knowing some history about the mirror can be crucial; for example, had the mirror previously been in a room with a dying person?  For novices, perhaps the easiest method to determine a mirror's hauntedness is to look into it at night by candlelight.  There's little guesswork in such an approach—you'll know!