Just under the name of the game, to the right of the ESRB rating (at least in the US, I don't know about overseas if they show a different rating...) will be a "DRM:" notification. This notification changes as you select different versions.
If this line reads "DRM: Origin" like on The Sims 3 you'll get a key to redeem on Origin.
It can also read "DRM: Steam" for EA games that are on Steam, like Tomb Raider. In this case, you'll get a Steam key.
If this "DRM:" is missing, generally that means you'll have to download it directly from Amazon using their downloader.
In the case of Ubisoft games where the "DRM:" line is missing, like the Far Cry 3 version without Steam DRM they'll generally give you a product key that you can put directly into uPlay and use their downloader if you so desire.