Antique Mirror

I have good news, "most mirrors" have the date stamped on their silvering on the back of the glass itself.

The invention of the silvered-glass mirror is credited to German chemist Justus von Liebig in 1835.[7] His process involved the deposition of a thin layer of metallic silver onto glass through the chemical reduction of silver nitrate. This silvering process was adapted for mass manufacturing and led to the greater availability of affordable mirrors. Nowadays, mirrors are often produced by the vacuum deposition of aluminium (or sometimes silver) directly onto the glass substrate.

Most of the ornate mirrors were done in the early 1900's and focused in Italy.

To this day, italy makes the best beveling equipment.
http://www.adeliolattuada.com/en/home.asp

There is no real instrinsic value to older mirror's since 1835 etc. The value of these pieces are often in the fram itself or the manufacturer of the frame/mirror if the artisan is known.

I personally avoid buying mirrors at estate sales due to superstition and hauntings/supernatural occurances reported with their sale. If it is a mirror held within the family it reflects the family.

But.. check the back of the mirror itself if it is not covered in paper or another covering, check for an artisans mark.