Best way is the conjurer spell contact other planes. It'll tell you if the item has a trigger phrase or special properties etc.

Sometimes you notice things on the thing if you look closely. For example, some inscription on the thing.

Sometimes the mob says a trigger phrase.

Sometimes your ally will tell you.

Sometimes there is a clue in the area.

Sometimes you just don't realise until later. I know I've sacced stuff when I had it and it seemed useless, only to realise when I didn't have it, what it did.

Also sometimes the clue is that it is hard to get and is high level for no obvious reason.