Magic Mirror

By means of this spell, the magic-user changes a normal mirror into a scrying device similar to a crystal ball.

The details of the use of such a scrying device are found on p. 141 of the DMG under the description for the crystal ball. 

The mirror used must be of finely wrought and highly polished silver of a minimum cost of 1,000 gp. This mirror is not harmed by casting of the spell as are the other material components—the eye of a hawk, an eagle, or even a roc, and nitric acid, copper and zinc (cf. 5th-level cleric spell magic font and 2nd-level druid spell reflecting pool). 

The following spells can be cast through a magic mirror: comprehend languages, read magic, tongues, infravision, and ultravision. 

The following spells have a 5% chance per level of the caster of operating correctly if cast through the magic mirror: detect magic, detect good/evil, message, and detect illusion. 

There is a chance of the target realizing he or she is being watched. The base chance for a target to detect any crystal ball-like spell is listed in the crystal ball item description, with the following additions: A cavalier has a base 5% chance of detecting scrying and a barbarian has a base 1% chance.