This is one of those rules situations that you don't like; It is not officially detailed anywhere, so it is entirely up to the DM to decide the answers for these questions when not covered by the particular effect in question.
I encourage you to go with a lenient ruling on the matter, such as letting the player declare a number of feet and a direction (i.e. "20 feet west" or "13-1/2 feet at a 3:1 away to right ratio" and the like) in the even that they can't see the path the effect would need to take well enough to more clearly say they move it from here to there - and simply have the effect stop if the desired direction and distance results in bumping into an obstacle of some sort that the spell effect can't pass through.