I had a go with a crystal pendulum last night, with excellent results! I am now going to treat myself to one at the earliest opportunity!
I figured out which was 'yes' and which was 'no' by asking it to show me a couple of times, holding it up with one hand, and having the other a few inches underneath it with an open, flat palm.
My friend then showed my how to locate the presence of spirit in the room. She uses a pendulum when she's on an investigation, and it works very well. Both of us had the same results, with it swinging wildly 'yes' over in the far corner of the room we were in. As soon as we moved away from that corner, they virtually stopped moving.
I also managed to dowse that she had a problem with her throat chakra, and that she needed strength and confidence in order to deal with a problem in her life, and speak her truth. She confirmed this was right, and that it had even manifested into a sore throat! I had checked all her other chakras before finding the problem with this one, and it always told me 'no' when I asked if there was a problem in that area.
I was using a clear quartz crystal pendulum, but my friend's was a heart-shaped dark purple amethyst. They seemed to work even better when we placed another large amethyst crystal a few inches underneath them as we worked.
Great fun!