The best defense against manipulation is an independent thinker. I have 3 boys (18-16-14). I've tried to instill in all my children to be cautious of your own thoughts and make sure they are yours.
A good simple test we would do is ask yourself "if I lived in the woods by myself with no one else around would I still want this thing or believe this idea?" That can start to give some perspective of where the idea stemmed from.