I suppose you could pipe a car's exhaust down one of the tunnels, let the car run for a while, and then look for new tunnels the next day or so. I've had fair luck running a hose down one of the tunnels and letting it run for a while. In my case about 15 minutes later I saw a sleek dark shape break ground and scurry for the neighbor's fence, with my dog in close pursuit.

   Dave