How can I easily confirm if an Andariel drop is a [bugged] quest drop or not?

Bugged andy doesn't drop pots, arrows etc. And the lowest item rarity is rare, so if all the magic items is failed set you should know it in a few runs.

If she drops 3 rare items and 2 magic items, and 1 of the 2 magic items has double durability but the other does not, is the drop a quest drop?

If the second magic item could have double durability, no. If it's an item that can't have double durability, it's probably a quest drop but I'd recommend another run to be sure.

Why wouldn't an item be able to have double durability? A magic falchion had regular durability, and a spetum had double durability.

I'll do another run as you suggest.

Well, some items just don't have durability and therefore you can't know if they're failed or not. A magic falchion with regular durability means that she's not bugged.

Ok thanks, so she's not bugged.

So is it possible for a magic item to spawn with double durability during a normal drop? What process is causing that, the same one which just happens less frequently?

Yes it's possible for non-bugged andy to drop magic items with double durability. Double durability can drop from any monster, it's simply an item that rolled set but for some reason the item couldn't spawn set (too low ilvl, or no set on that base exists). Then it rolls double durability instead. Since bugged andy can't drop lower rarity than rares, you know that all magic items should be double durability due to failed set.