I posted this in the bug reports forum, but my post quickly sank several pages back without a reply. Basically, there is a way to ascertain whether the opposing team is fighting vilemaw without vision of the vilemaw area.
1. Make sure your team has no vision of the vilemaw area. This bug can be reproduced with your entire team in the fountain.
2. Move your camera over the vilemaw area. With the fog of war it will be blacked out.
3. The enemy team begins to fight vilemaw.
4. Listen to the sound effects from your client. You will distinctly hear the sound effect from vilemaw's ground slam as the enemy team fights vilemaw. You will hear every ground slam.
I don't think that this is intended behavior since Twisted Treeline is a "wardless" map. It certainly doesn't work this way on Summoners' Rift when a team is fighting dragon or baron. Every time I back I move my camera over the vilemaw area if I think the enemy team might engage him, and I instantly know if and when they do. This probably isn't very fair since I doubt many people realize this behavior exists.