All capital ships and probes have scanners. Scanners allow you to see fleets in nearby space. How near you can see them is determined by the ship's scanner level. The scanner level of a fleet is that of the highest level present in the fleet.

A scanner level of 0 means you can only see fleets located at the same exact coordinate as your fleet, and you will not be able to gather any additional properties.

In open space maximum range is 3,000,000 km per level, or 750 coordinates. When deployed as a listening post, it is 10,000,000km per level. At the maximum range, you can not determine any details about a scanned fleet, other than it exists, and if it is moving, its direction and speed. If the flagship of a scanned fleet is a command frigate, it may distort your scans and make it even harder to detect anything at a distance.

From local space you cannot see out into global space, or vice versa. Inside a system, scanner range is reduced to 1/10th. You can not detect fleets at orbits or colonies from local coordinates.

From a colony, a scanner level of 2 is required to scan orbital positions, from orbit a scanner level of 3 is required to detect fleets at colonies. Level 4 is required to scan local coordinates from orbit.