How do I know if a Legendary item can be obtained at Kadala?

Reaper of Soul has introduced with Kadala a system of gambling. I know that you can get legendaries from her but I don't know if every legendary in the game can be obtained by gambling.

How do I know if a legendary can be gained through gambling or not ?

We can distinguish 5 kinds of legendaries :

• "Regular" legendaries : can drop anywhere in the game, at any difficulty
• Legendary crafting Plan : can drop anywhere in the game
• Torment-only legendaries : can only drop in Torment 1 difficulty and above
• Legendaries Potion : can drop in Torment 1 difficulty and above
• Horadric Cache-only legendaries : can be found only in a Horadric Cache

Among those 5 types :

Kadala can give you only Regular ones and Torment-only ones (since patch 2.0.5). Since you can gamble in every difficulty, you can hypothetically obtain a Torment-Only legendary in Normal difficulty by gambling (though it requires some luck).

Kadala can neither give Plans nor Potion (since she does not sell any of those). She can't neither give Cache-only legendaries :those are still reserved as a reward for completing bounties.

Short answer : If you want to know if you can obtain a legendary at Kadala, just look in which category it belongs. If it's not a Plan/Potion or a Cache-only legendary, you can get it at Kadala.