
One thing you can look for are termite tubes (mud tubes) like the one in the picture to the left. Subterranean termites travel in these tubes to get to their food and nesting sources.
These tubes are more easily made with sandy soil, which is why you tend to find more termites in sandy areas. While this is not the only thing you should look for with termite activity, as a home owner it is one of the easier things to spot before the issue gets out of hand.