Mold in Unusual Places
Mold can be normal in some rooms, especially around your shower or bath enclosure. However, it might mean something more serious than humidity if you have mold on bathroom walls or between your baseboards and flooring.
Plaster and drywall can very easily break down when exposed to the high humidity and moisture that leaky pipes cause. Even if you don't see any discoloration, there may be a leak that is dripping water down through the space between the walls and causing damage.
Unexplained Water Usage
If you've lived in your house for a while, you probably know how much water you use on average. If your meter starts consistently running over that average, you could have a leaky pipe. If you suspect a water leak and have access to last year's bills, checking them against each other is a good place to start. Or, if you want some more tips and information, check out this site about leak detection in Pacific Grove.