Returning player leveling a new char. How do I check my main story progress???

[Terrini]

You can go into your journal ("J" on pc) and check completed quests in main story. You can turn in new quests post 50 on any appropriately leveled job.

[Ooshima]

The easiest way is to remember the time you went off. If you went off say Patch 2.1, then check out Patch 2.1's patch notes against your Journal to see if you completed everything, if not, follow it through and then move on to the next following patch notes. The patch notes gives you the starting points (coordinates and name of the NPC) that gives you the relevant quests to move on.