If they picked up someone's/your's then yes, it is a 100% drop rate on all of it. If they spawned with it on, it is a small chance to drop one or more of the pieces. I rarely see any dropped, and if at all, it is always one, but it could potentially be more.
I've seen a diamond armor skelly once, but it took about 500 skellies till we got one (a friend and I were making this castle mob siege thing on a private server.)
Usually you can help tell because if a piece of their armor is enchanted, they usually drop that piece of armor with the enchantment, it happens to me all the time, but sometimes it doesn't drop the enchanted stuff.
