Child custody laws in Pennsylvania

My ex-wife and I are in the middle of a bitter divorce in which our young son is the main focus. We had come to an agreement that we would both retain joint custody. However, her attitude has since changed and I have a feeling that she may not only pursue full custody but, may also restrict my visitation. Is there is any form or actions I can take or start so that if anything happens where she tries to take my son and not let me see him, I can stop it. Are there any actions I can take to have things ready to request full physical custody as well?