Look for original glass. Antique glass is fairly thin, so thick glass without "exfoliation" is suspicious. "Place a quarter sideways against the glass," counsels Leslie. "The thinner the glass, the closer the quarter appears to the mirror."
And beware of a crystal-clear reflection. "Antique glass was often resilvered several times in its lifetime, and minor mottling gives glass a softer look," opines Leigh. For collectors hoping for a return on their investment, "original glass is key," he says. Check around the mirror’s edge, where slightly jagged glass held in place by a reeded ebony liner (or behind the backboard) can sometimes be seen.