I don't entirely understand the problem. There are passenger missions ("transport such and such") that go from A to B, in which case you only need to drop them off at B. Then there are (sightseeing) missions that go from A through B, C and back to A - in which case it will show you destinations in order which it wants you to visit them, and as you visit each destination it would show you the next one. Once you visited B and C it will direct you back to A. Going back to A without visiting the other destinations wouldn't complete the mission. Just select the mission in question and press open galaxy map, and it would show you where it wants you to go next.
I usually avoid the latter types since I can never find different passengers that go to the same places so you end up having to drive all over the place.
Edit: Yeah I get what you mean now. No, the game doesn't tell you that until you get all the objectives out of the way. Maybe there's the mission giver's info somewhere that would give you a hint.