Hi, I've started playing the Legendary Edition on the XBox and recently downloaded DB, DG and Hearthfire.

Obviously various npcs are now being killed off by Vampires as part of the DawnGuard story. I've found Pantea's Flute on an earlier trip but can't find Pantea Ateia now that I'm doing the Bard's College radaint quests. My gut feeling is that she became vampire food.

How do I find out if a particular npc is dead on the Xbox?

check the Hall of the Dead in Solitude for her.