it is known -as per my view-by exprience-if he or she is not telling you the truth for the first time then there must be come cogent reason may be to protect some body who could be your known person-normally a best friend will not lie
Duct tape them to a slanted wall and interrogate them. Make sure they are forced to listen to justin bieber on repeat. Every time they say something shout LIAR and force their mouth open and put some soap in. when they ask why you just did that tell them you are cleaning the lies from their mouth. Put on a care bear puppet show and force them to watch it. Gently run dart points or knifes across their face while wearing a Halloween mask to freak them out. Each time they beg you to stop, Justin Bieber gets put on louder and an older song so they just get worse. Once they are crying you will know they are telling the truth. HAHAHA JK
If this person is your best friend, why not just ask? If your friend becomes immediately defensive that you are questioning their truthfulness, this is a clear indication your instincts are correct and you can then decide how to go from there. Communication is key to any successful friendship, relationship or marriage. Good luck...
first of all you should know your best friend that well. second see if they have strange looks or get nervous that may help. also another thing that's important is if the person changes the subject, which means they know something good or bad it may surprise you.