I'm looking at old houses, hoping...

Start from the basement and check windowsills, foundation walls, and crawl spaces, looking for the discarded wings of reproductive termites (whitish and opaque) or for the half-round tunnels that termites build from the ground to their wood supplies. Worker termites are blind and sensitive to air and light, so these tunnels protect them. The tunnels may be especially evident on brick, stone, or cement walls, or sometimes on metal pipes that extend vertically. Look closely at the joints between the basement floor and the walls, especially if they’re cracked. Check the outside of the foundation, as well as any trellises or fences that connect to the main building. Look around the cellar bulkhead, if there is one. Anywhere wood comes in close contact with the soil is a potential gateway for termites. Some builders install copper termite shields to act as a barrier between the earth and the wood, but older houses are unlikely to have these. Observe whether building lumber has been discarded or buried anywhere nearby, or whether the crawl space has old pieces of scrap wood lying around. These are open invitations for termite invasions. With permission from the owners, use a penknife or awl to probe any wood that looks suspicious. If you can extend the blade half an inch into the wood with moderate hand pressure, something is wrong. It may be termites, or it may be wet rot or dry rot, fungal conditions that also cause wood to deteriorate.