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I just double checked the code, and the behaviour is:

- Drop on aligned altar: bless (burns some piety)

- Drop on non-aligned altar: curse

You can identify item statuses via scrolls of identify, or by playing a PC with the Lore skill (merchant, minstrel, pilgrim, sage, wizard, witchling).  The Lore skill has a one-time chance to identify an item on pickup.