When you first log on, I believe the update refreshes you on where your at in the main story line. You can also look at the rewards. Main story line almost always gives large amounts of exps. The side ones give practice points, companion points and companions etc.
For all of you being smart to his original question I have this to say... I too would like a small indicator. I don't think it's too much to ask for a small legend icon that determines whether it is a main quest or side quest. Yes, we can easily track them by hitting Q, but that's not what he is asking.
Would also be nice to see the (suggested) level of each quest somewhere. Recently I found that I completed a lot of side quests (with level 27/28 mobs) before I did some other older (level 18/19) quests. Would had been more fun if I did them in order, the first lot wouldn’t had been so hard and the others not so easy J
Not a major problem by any means, but would be nice to see some indication.