How Do I Know if someone is lying??

Hey there!!
so the other day a friend of mine told me something and I just cant believe what he said.... really is it that hard to believe in your best friend. Anyway I really feel like he´s lying so I dont really know what to do cause I hate liars........and I mean he is my Best Friend so please if someone could give me advice in how to know if my friend is a liar or not cause I dont want to mess things up

So yeah add me message me or comment me I will be happy to read any of your suggestions and im thinking on makin a story but I dont know how to start so having a bit of trouble there. So yeah summer started for me so im happy but bored and hungry hahahahahaha did i said im starving too so yeah now im leaving please comment me cause i dont want to mess my friendship up soo yeahhhh im leaving bye!!!!