The eyes and ears of a star ship. They come in two broad classifications, Active and Passive. In general, military sensors are far more sensitive and have longer ranges then civilian sensors.
Cylon scanners work on principles similar to Colonial systems.

Passive Scanners do nothing but listen and watch. They send out no signals or scans to locate objects. All the information they gather is derived from detecting and measuring the signals emitted by other objects or reflected from environmental sources.
Every aspect of a ship’s functions gives off some type of energy signal, from faint to strong. By listening to and analyzing the signal a great deal can be learned about a ship.
Visual scanning, using telescopic devices to magnify a target, is another type of passive scanning. So is monitoring communication channels. Spectroscopic analysis of naturally reflected light to determine material composition is also passive scanning.
The range of effectiveness depends on the sensitivity of the gear, background energy and the size and energy level of the subject being observed. In general passive scanners have ranges far greater then active scanners. Ranging from 3,000,000 to 6,000,000 metrics (2,250,000 to 4,500,000 miles)
The natural energy emitting from a star or planet can serve to illuminate a subject to passive sensors or conceal a subject. Interpretation of passive scans is a highly regarded skill and a good sensor operator is a valuable asset.

Active Scanners project a variety of energy beams or waves at a target or area and analyze the return signals to learn about the target. RADAR and LADAR types are the simplest active scanners and are used in navigation, but far more sophisticated types able to detect chemical elements, biological traces and life forms also exist. Active scanners can locate and identify ships, providing precise details of mass, features, course, speed, composition, energy sources and the presence of life on board.
Standard Civilian Active Scanners have an average range of 800,000 metrics (500,000 miles)
Enhanced Civilian Scanners (Common in the Rag Tag Fleet) have an average range of 1,000,000 metrics (750,000 miles)
Standard Military Scanners (Found in Vipers and shuttles) have a range of 1,600,000 metrics (1,000,000 miles)
Enhanced Military Scanners (Found on large military vessels) have a range of 2,000,000 metrics (1,500,000 miles)
In general there is little skill needed to interpret an active scan. Most of it is done by the computers that process the scans return signals.

An active scan is immediately detectable by passive sensors at twice the range of the active scan.