How to figure out if your best friend is Lying
Friendship is a bond that can never be broken regardless of the situation or betrayal. You can steal all your best friend's money and their significant other and still be bonded with them for life. The connection never goes away even if you've been estranged from that person for many years. Once you bond with a person, the friendship is like a marriage without the added complications and living under the roof with that person. Thankfully, that's what marriage is for.
A best friend is defined as someone who knows you the best. That person is the one who knows all your deepest darkest secrets. They're the person who can either shield you from harm, or stab you in the back when you're not looking. Who needs a best friend when your worst enemy is in your midst? Ironically, friendship is like a war where both sides once rejoiced in peace are now at each other's throats. The friend war is much smaller in scale and there aren't any guns or explosives within thousands of miles. No side ever wins and everyone loses. All that's for certain are the casualties and collateral damage involved during the coarse of the epic battle.
How do trust your best friend once they've stabbed you in the back or in the chest? Do you take that person for their word or write them off completely when the damage has been done? Decide if your best friend is worth a second chance by going out to lunch with them in a public setting to avoid the risk of a big scene. Ask them point blank the truth and what they're willing to do earn back your respect. Test your friend without making them go through a ridiculous game show like obstacle course. It's your life, not Survivor Island. Demand complete honesty from your friend to determine if they're lying to you. Since you're the person that knows them best, you can easily figure out if they're telling the honest truth or spinning another web of deceit. Figure out what's really going before taking the next step.
Taking the direct approach lets you and your best friend know what's really going on with your friendship. Don't go down to their level by promising them everything will go back to normal when that's the furthest thing from your mind. Never plot revenge when neither one of you wins. Remain objective even if your friend is ticking you off to no end and you want to punch their lights out. Be the bigger person and keep a cool head. Pour a glass of ice water on your head if you have to. Just stay calm and don't resort to violence to get their attention.
Ultimately, your friendship has been forever damaged but it's to you to determine if it's repairable or not. The ball is in your court to come to either a swift resolution, or let your best friend sweat as long as possible. Hey, you need to get your revenge in there somehow.