When we had termites, we first noticed the sawdust like powder around the house. That's the powder the termites create while munching on the wood. And then we noticed that there was a leaking pipe under the house which really confirmed the termites for us. Some termites only eat moist wood because that's how they get their water requirement. Some termites can feed on dry wood because their system works differently. Those termites are only found in certain parts of the country though.
So if there are leaking pipes around the house, it's a good idea to get them fixed and check for other signs of termites.