Every starship has drive systems that propel it through space. The drive energy is noticeable because of the photons that are emitted. Not only can it be seen, just by measuring the light and it’s spectrum, the model of drive system can be identified. If a drive is modified, it may give off a unique signature but even then a guess could be made as to what the original model was.
Just by observing the drive’s output and the vessel’s speed, a skilled observer can calculate the mass of a ship. Just knowing the mass of a ship can give away what model of vessel it is, whether it is carrying cargo and possibly how many crew are on board.
Controversy
Some feel that any ship that enters a system would automatically detected and there would be no way to hide in space. Personally I don’t see evidence for this thinking. Despite having many telescopes at their disposal, NASA could not see the Cassini space craft or it’s thrust plumes even though it’s the size of a small school bus. If they could have, in any way, recorded the probe’s decent into Saturn’s atmosphere they would have gladly showed them. This included a 100% thruster burn as it entered the atmosphere.
The problem is resolution and luminosity. The distances in a star system are enormous and that makes imaging a object smaller than a large moon difficult. The more powerful the drive, the further away it could be observed because of the light it emits. Some drives, like plasma drives may be harder to observe. Chemical and especially nuclear drives would be observable from much further away but at eight light minutes away (1 Astronomical Unit) a telescope would still have a hard time picking these drives up.
In the Jump Temp universe, a large portion of the outer solar system is easily accessible via jump drive. A ship jumping around the outer regions of the solar system could be easily hidden by jumping behind various stellar bodies.