Rebel:

Pull of 3 or 4 squares of paper towel and lay it out behind your toilet under the tank.

Not only will the dark spots or areas on the paper towels make it easier to see condensation dripping off the tank, but the embossed pattern on the towel will relax if it gets wet. So, even if the towel gets wet and dries out again, the flatted embossing will prove it was wet, but dried out again.

This is totally off topic, but it's interesting...

Take a paper towel and put a drop of water on it.
Notice how the wet circle is darker than the surrounding white paper towel?
Now, hold the towel up to the light.
Notice how the wet circle is now brighter than the surrounding paper towel?
Why is that?
And, the answer to this question will explain why your blue jeans are darker when they're wet... AND how wet t-shirt contests work. That is, why wet cotton is more transparent than dry cotton.