A player controls 6 Mountains and has an Inferno Titan and a Lightning Bolt in hand. They begin casting the Titan by placing it onto the battlefield, clearly announcing that they are playing it and tapping some of their lands. Then they realise that they only have 6 lands, not 7 as they thought and they want to cast the Lightning Bolt aswell. Do we force the player to pay for the spell? If we don't, what is the infraction? The player is committing a GRV intentionally in order to gain an advantage, so it would have to be Cheating. So we are effectively forcing them to pay?
What if the player controls only 5 Mountains, but has a Llanowar Elf? I presume this would be the same answer.
What if the player controls only 5 Mountains, but has a Simian Spirit Guide in hand? Wouldn't the ruling on this reveal information the opponent should not have access too?
What if they control 5 Gruul Signets, no Mountains but they have a Simian Spirit Guide in Hand? Here, the opponent would be safe to assume the player has the Spirit Guide anyways unless they assume the player made a mistake.
What if they control no permanents, but have 4 Simian and 2 Elvish Spirit Guides in hand? In this case, the opponent can very safely assume the player has enough mana regardless of ruling.

What I'm trying to get at is: We are kinda forced to force a player into certain plays, but we obviously can't do this if the player has no chance of making that play. Is it fine to reveal information the opponent should not have access to this way?

Closed pending official answer

Martin, so much depends on being there; if a player starts to take an action but changes their mind before gaining any information (including an opponent's reaction), judges are likely to allow that.

We don't force players to activate mana abilities. That's why we can ignore “must attack” requirements when there's an additional cost to attack - even if there are untapped lands that could be used for that cost.

d:^D