On
LOOT_READY you can iterate from 1 to
GetNumLootItems() and call
GetLootSourceInfo(index) to get the GUID of the unit being looted.
If you're looting a chest, the GUID will start with the word "GameObject", and will contain a unique identifier for that particular object as the 5th numeric field; for example the
Felbound Chest from Krosus has an object ID of 248513, which you can see in wowhead's url, so its GUID will look something like "GameObject-0-0-0-0-248513-0".
I'm not sure whether you want to match loot to a specific boss or just want to know whether a "boss" is being looted, or if any high level boss-like loot has been found, because there are a few different ways to go from there.