how do you know that your friend is backstabing you?

This is a tough one. These people are precisely called backstabbers because their victims don't know they're doing it. Like what they say, hat goes around, comes around. You would know eventually. Don't go on suspecting everybody about this, just make sure, instead, that you are surrounded by true friends, who will let you know in case another friend is actually backstabbing you.

you will just know by your instincts. if you feel that something is not good or hear somebody messing your life, try to investigate.