I have two questions about discarding cards while casting a spell:
I control a Chromatic Sphere and Words of Waste. I activate the ability of the enchantment. Then, I start casting a spell, sacrificing the Sphere. Will my opponent discard the card face up or face down? If it is face up, it creates a way to have a free information about a card in opponent's hand, if I finally do not cast the spell.

Now, I also control a Waste Not. If my opponent discards a land, is the trigger a mana ability, as it creates mana and triggers from a mana ability?
Let say I want to play a Thoughtcast and need to have my star to get the affinity and need my opponent discard a land to get BB. I need to go until the discard revelation to know if I can cast my spell or if I need to move rewind the process.

Bonus Question (to add some craziness): I control Chromatic Sphere, my opponent has a Waste Not and a Chains of Mephistopheles. If I sac my Sphere on my turn during the cast of a spell, discarding a land, will the Waste Not's trigger be a mana ability for the opponent?

Edited Mikaël Rabie (Jan. 31, 2016 05:36:05 PM)

Originally posted by Mikaël Rabie:

I control a Chromatic Sphere and Words of Waste. I activate the ability of the enchantment. Then, I start casting a spell, sacrificing the Sphere. Will my opponent discard the card face up or face down? If it is face up, it creates a way to have a free information about a card in opponent's hand, if I finally do not cast the spell.

The card is discarded face-up. However, if you start to cast a spell without intending to finish simply to increase your knowledge of your opponent's hand, you're probably gonna have a bad time. As has been previously explained in a similar case, intentionally taking illegal actions to gain advantage is Cheating, and will come with a DQ.

Originally posted by Mikaël Rabie:

Now, I also control a Waste Not. If my opponent discards a land, is the trigger a mana ability, as it creates mana and triggers from a mana ability?

No, it is not. Waste Not's ability can never be a mana ability, since it does not trigger from the resolution of an activated mana ability. It triggers from a player discarding a card. The fact that in this case the discard happens during the resolution of a mana ability doesn't matter–in order to be a mana ability Waste Not would have to trigger off of the resolution itself, not something else that just happens to occur during that resolution.