Scrying

Scrying was the magical ability to use one's senses to experience events far away.

Magical sensor
Many divination spells of the scrying subschool created a magical sensor that sent the caster information. The sensor had the sensory acuity of the caster and acted as though it were a separate, independent organ. Any intelligent creature could notice an active magical sensor. The sensor could be dispelled.

Limits
Scrying could not penetrate lead. Several magic spells, antimagic field, mind blank, nondetection, and wardmist could block scrying. Magical items such as weirdstones could also block scrying. The semi-precious stone tchazar could blur the scrying of anything close to it.

Aids
A chrysoberyl gem (aka "cat's eye") could be used for short-range but very detailed scrying.