Scrying

Scrying is a magical practice that involves seeing things in a medium to obtain knowledge of things that are happening, past events or the future. It's unknown if, in the show, this ability is only possessed by Seers and those who have the Sight, or it can also be performed by every gifted warlock or witch. Anyway, Seers have a natural talent for scrying and can easily see things with their powers.

There are many different ways to perform this, depending on the nature of the medium.

Hydromancy
Hydromancers can see things in a water medium. The High Priestess Nimueh had this power and she used a scrying fount to magically view events in various locations. To use this power, she let a few drops of her blood (from a cut on her finger) drip into the fount and chanted a spell. Although not subtitled in the DVD, it's possible that this spell was: "Diegol cnytte, gewitte me yst, þa tacnian me yst þonne ieceþ sicle. Diegol cnytte, gewitte me yst, þa tacnian me yst þonne ieceþ sicle. Swilte ar ond calan, drædan morðor, to Camelot he cymþ" and "Diegol cnytte, gewitte me yst, aliese hine, to Camelot he cymþ". To change the images she shook the water surface. It's unknown if Nimueh could also hear noises through the scrying fount and, although she declared that she had the power to see into the future, if it could show her events that hadn't yet come to pass (The Mark of Nimueh, The Poisoned Chalice).

Merlin unsuccessfully tried to scry the vial the Fisher King had given him with the incantations "Geriht læc me", "Fultume me", and "Geondlihte gesweorc min" (The Coming of Arthur).

Crystallomancy
Crystal-gazing (also crystal-seeing or crystallomancy) is a form of divination or scrying achieved through a trance inducted by means of gazing at a crystal. Morgause, High Priestess of the Old Religion, used a crystal to keep an eye on Arthur, Uther and Camelot. She was probably trained by the High Priestesses in this skill and she was taught different spells to focus her powers ("Hider eft funde. On þisse ne middangeard" and "Ætíe mé þá þé ic séce") (The Sins of the Father, The Tears of Uther Pendragon).

The Crystal of Neathid and the other crystals from the Crystal Cave can show future events, even if the sorcerer who looks into them doesn't want to (The Witch's Quickening, The Crystal Cave).

Oneiromancy
Oneiromancy is a form of divination based on the interpretation of dreams. In the show, Seers have involuntary dreams in which they see the future. A seer's dreams can be stopped from not making him sleep at night if an enchanted bracelet is worn, but this only stops simple visions and not prophecies. Morgana Pendragon possessed this magical ability.