Main scenario quests are those with the "flame" meteorit frame around quest icons above NPC's heads and on the map. You need to follow the chain of the quests to unlock them in sequence - look here and check for (missing) Prerequisite Quests - you can trace the "chain" there in both directions. You can see your latest completed (main scenario as well as any other) quest in your Journal under Completed Quests.

Hope this helps