Working with a pendulum is one way of communicating with one's higher self and the spiritual realm.
Your higher self can only answer questions that pertain to you and not another person. And when you ask a particular question to your higher self, you need to ask with a clear and open mind.
If you are expecting to have a particular answer in your mind, your mind can control your pendulum.
So the answer to your question is "no". A pendulum will not be able to reveal whether a particular ex will be coming back into your life.
It is up to the "ex" in question whether he or she wishes to return.
On the other hand, if two people are supposed to be together it will happen.
In the mean time, I suggest you work on the issues within yourself that were revealed to you regarding a particular relationship.
Always remember that the other partner in the relationship is your mirror revealing parts of yourself.
Sending healing light and love, tanya Tkach, Spiritual counselor