[Alneyan]

Planets do not detect cloaking automatically (well, unless using a modded facility to do that). You will need a ship/satellite deployed in space.

There is no range factor involved: if they are in the same system as your sensors, you will see them. All your other ships will also see them, even if the sensor is at the other end of the system, and even your partners will see those cloaked ships (including yours by the way).

So basically, one sensor per system and you will be fine... unless this sensor is destroyed, of course.