What's the best way to store super-firm tofu? Does it need to be in water? How long will it last? How do I know if it goes bad?
It will keep for up to a week in the refrigerator if you cover it with water and change the water every day. You can freeze it for about 3 months. If you drain it before freezing, it will have a meatier texture.
You'll know it's not good by how it smells. It will smell rather sour and unappealing. It probably won't hurt you to eat it if it smells a bit off, but it won't taste very good.
How do you know it it goes bad? The water will be syrupy, it will smell foul, there may be pink mold on it... you will know! Follow drbabs advice. Another approach is to take what's left of the block, cut it cubes and marinate it in a vinaigrette or teriyaki sauce -- then use it!
Stored in water and changed once a day super firm tofu can last a few weeks. If you add salt it will extend its life, but will also impart a salty flavor. The nose is the best judge of whether or not the tofu is bad. When it smells bad it will most likely taste bad, and that is a good time to toss it.
When I asked my Japanese friend years ago how I would know if tofu was going bad, she said, "Do you know that very bad smell?" You'll know. Go with submerging in water and changing the water every day if possible and use it as soon as you can. My problem is always forgetting to change the water if I try to keep it too long.