You might find: a horizontal line running across the glass. Old mirror glass was limited to the size glassblowers could handle, so many are made up of more than one piece. Typical proportions allow for two-thirds of the mirror below the line and one-third above.
Steer clear of: a new "aged" mirror in an old frame. New glass is not uncommon, as glass would frequently break. But check the seller's description and make sure you're paying for what you're getting. And of course, as with most things, a mirror shouldn't try to be what it's not. New glass made to look old doesn't have the distinctive muted and mottled sparkle achieved over hundreds of years. Old mirror glass reflects light in a magical way. Also watch out for added embellishments to the frame and significant changes to the mirror's proportions.
