Like any other abuse.
Is there one person "in control" of another or others? Are there "rules" that when broken you are punished for? Is Spirituality used for gaining control over a person, their bodies, their finances?
Does the Spiritual path require that you do not have contact with "outsiders" and does it frown on asking questions? Does the teacher/leader claim special knowledge from the religions deity(s)? Does the leader/teacher claim to be above the law and ask his/her followers to engage in illegal activities?
These are but a few warning signs of possible Spiritual abuse. These abuses can happen in ANY religion or spiritual path.