Know Basic Bible Teachings
No one can know all the false teachings of all the cults, but Christians can learn the Bible well enough so that they will recognize cultic doctrine.
Bankers are trained to recognize counterfeit currency by studying genuine dollar bills. Thus, bankers will recognize a counterfeit bill when they come across it. The more a person understands the Bible, the better that person will be able to recognize cultic errors.
Scripture warns about spiritual deception. Galatians 1:8 speaks of "a gospel other than the one we preached to you." Second Corinthians 11:4 warns us to avoid those who preach "[another] Jesus... a different spirit... a different gospel."
If a Jehovah's Witness says that the Holy Spirit is a force and not a person, show him or her passages that demonstrate that the Holy Spirit is a person: 1 Corinthians 2:11-12; 12:11; Ephesians 4:30; Romans 8:26-27.