Scrying

Scrying is the magical ability to find a person or an object through the use of a crystal and a map. It is one of the four basic abilities possessed by witches and warlocks. However, Leo Wyatt has also used this power when he was an Elder.

Description
All one has to do is hold the crystal pendant over a map of the area where it is thought the lost person or object will be. The more violent and vigorous the swing of the pendant, the closer you are to the object on the map coordinates, like a "warm or cold" method of looking for something i.e. the more vigorous the swings the "warmer" one is. The pendant will eventually pinpoint the exact location of the object or person by pulling down on to a spot on the map after a certain amount of time.

For a scry to pinpoint the exact location of something, usually an item of that person or demon is needed, such as clothes or blood. Piper used the eyes of a snake to scry for Saleel, a reptile demon, so it is assumed that the object can also be related to the thing you are scrying for. Scrying is one of the four basic powers a witch/warlock has. Elders can also use this power.

Amethyst is said to work best as the most powerful scrying substance.

Alternate Method
n Season 4, the Charmed Ones come into contact with a Zen Master and his student/daughter An Ling. The latter tells the Charmed Ones that her father could use a surface of water as a looking glass into other worlds. This seems very similar to the real world method of scrying and may be the Zen Master's version of it, especially as An Ling said it was a power and one she could learn as well as her father.

In the Season 6 episode, The Power of Three Blondes, the Stillman Sisters use a bowl and some sort of powder/liquid substance to scry for and spy on the Charmed Ones.

In Season 8, a new witch named Billie learned to scry with her computer using a GPS on-screen map and a large quartz crystal attached to her mouse, moving it over the mousepad to move it around the on-screen map. When the target was located, the on-screen map would put a red square over the area and the crystal would stick fast.