Luke doesn't actually know where he's going, but he does, eventually, pick a spot that's relatively out of the way and has panels and things to hack into, because they can't blindly wander around the ship forever.

(As for his hacking skills and whether he is good enough for this, well: the narration says consider how he was in the first season and add many years of practice with Mr. Smith.)

After a bit of fiddling, he comes up with a map of the ship, and the life forms on the ship. The ones not meant to be there are in mauve, and one particular mauve one that Luke assumes is Ross is only a few corridors away.