A good way to tell if a close friend talks behind your back is if you tell that friend something confidential and another mutual friend doesn't necessarily repeat it back to you but bring up that issue in a question and answer kind of way.
You know you've never had this kind of discussion with this mutual friend which means it slipped out of your close friend's mouth, it's a clear indication your close friend has been talking behind your back.
READ: How to persuade a friend to leave their unhealthy relationship
Another indication is if a friend looks at you in a funny way, especially when you walk by it's more like they know something you don't. Sometimes, they stop talking and act like everything's okay when you come around but deep down, you know something isn't right.
Watch relationship expert and life coach, Marie Dubuque explain better in this video.