How do you confront someone if they're lying ?

their reaction and behaviour

You can confront someone if they're lying by first observing their actions and behavior to judge how they are going to react. Then when you talk to them say something like " I feel like you have broken my trust by saying that you were sick but then I saw you going out with Suzy. I don't feel hurt by this" or whatever pertains to your case. Use "I" statements instead of "you" statements so the other person will listen to you. However be assertive and make it clear that you are unhappy, but try not to come off as threatening or overbearing as they will shut down, deny they were lying, and not listen to you.

Hope this helps!  :)

Explanation:

you! go to the gym or i will kick you out of here to the gym