Check your quest completion during the event.
The Unstable Prism
Legacy Script - Wrath Classic
/script print(IsQuestFlaggedCompleted(40173))
Modern Script - Retail WoW
/script print(C_QuestLog.IsQuestFlaggedCompleted(40173))
- TRUE if completed.
- FALSE if you have not yet completed it during the current event.
This is a basic script to check quest completion. Run it from your chat window as you would any other command in the game. Similar, albeit more advanced scripts, exist to check for NPCs killed each week such as the World Bosses in Draenor.
You can also substitute the 5-digit quest code above with other quest codes found on Battle.net and here on Wowhead to check their completion status.