In my highschool biology class we did an experiment using mustard powder to find worms. Here's how it works.
Take a small amount of mustard powder,(which can be found at most grocery stores),and mix it with water. It should form a runny liquid. Find a spot where you think worms will be and pour it on the ground. after a while worms will start to emerge.Rinse off the worms and you're good to go!
What happens is that the worms are irritated by the mustard and wriggle to the surface in panic.