I find strange that this question didn't come up a lot of times already. If that's the case, I read the FAQ and i couldn't find an answer, I'm sorry.
I play through different terminals and I always have the quest database messed up since is saved locally in each terminal.
Is there a way to keep in sync the files? for example a way to keep the settings in a dropbox folder?
Of course the best thing would be a 'sync' button ingame that scans the completed quests on the server and updates the local files.
Thanks for the hard work, still the best addon ever made.

You could try installing your WoW Client in a directory that can be monitored by DropBox or OneDrive and synch ONLY the WTF folder.

As far as asking the server for a list of quests you've completed:
questsCompleted = GetQuestsCompleted([table])
that functionality wasn't added to World of Warcraft until patch 3.3.0.

I solved by creating a junction to the original path using few batch strings

mkdir "%USERPROFILE%\Dropbox\WoWSync"
move "%WOWPATH%\Interface\AddOns" "%USERPROFILE%\Dropbox\WoWSync"
mklink /J "%WOWPATH%\Interface\AddOns" "%USERPROFILE%\Dropbox\WoWSync\AddOns"
move "%WOWPATH%\WTF\Account" "%USERPROFILE%\Dropbox\WoWSync"
mklink /J "%WOWPATH%\WTF\Account" "%USERPROFILE%\Dropbox\WoWSync\Account"