Discovering a solar system that cannot be seen by the naked eye on earth seems like an easy task with telescopes (e.g. the hubble telescope). I'm guessing, that our sun is the reference point for the bearing of the telescope at the point of discovery of a solar system.
But how do you find this solar system again? F.e., after discovering a "new" solar system, a year passes by, and there is need to know in which "direction" this solar system is in relation to ours.
A bit off-topic, but a reason as to why this question might be important: In science fiction there is often the possibility to travel faster than light, reaching other solar system quickly - or for the sake of this question lets assume, we would be able to travel there too in a timely manner. How would they/we know where to go, since the universe is ever expanding.