Using a pendulum is one way of assisting choice, especially if you are learning to trust your intuition. Start by buying a decent pendulum and first of all find your yes and no by letting the pendulum swing clock-wise or anti-clockwise. Ask with your mind which is yes and which is no, for you. The pendulum will either go one way or another. Try it a few times until you are confident you have found your yes and no. This is an empowering way to connect to your own intuition. When you are confident hold the pendulum over your crystal or over the word of the crystal in a book and ask in your mind a question about the crystal.
This is a very important point and like with any intuitive approach the question you ask will be just as important as the answer.
As an example ask “is this crystal right for meditation for me at this moment in time” The key here is to ask directly about the present time and the usage of the crystal. The clearer you are the clearer the response. As with anything practice and patience are the answer to good dowsing skills.