IsQuestFlaggedCompleted returns true if you have completed/finished the quest (turned it in).
Take a look at the Quest functions and the way Blizzard handles it:
http://wow.gamepedia.com/World_of_Wa...uest_Functions
http://www.townlong-yak.com/framexml...QuestFrame.lua
http://www.townlong-yak.com/framexml.../QuestInfo.lua