If they travel it enough they will leave a trail thru pine needles from what I have seen. I mean I doubt you will track one thru it, but if it's a well used trail you should be able to see it. Your pretty screwed with the water. You could look to see where they enter and exit the wet parts though. That should be pretty easy to see.

The string idea works pretty well too. Sort of like a really low tech game camera. I think that is how they used to be triggered even. Before motion sensors and crap.

Paul