How can you know if a sales person is lying to you or not?

Their body language, facial expressions and eye contacts.

listen to his presentation carefully and after he is done ask him to repeat what he said again as you didn't completely understand. if he starts again listen carefully, if he changes anything you will know.

people do two things if they lie ,they cannot remember what they said the first time,so they will make new lies.

or if he exactly starts talking like a taped interview this means he memorized the presentation and that is not good either.

if you talk and he doesn;t listen, kick  him out. he is no good.

Verify all the facts, the salesman has given from various fb blogs and feedback from those who used and experienced those products.

Everybody is not a policeman, to catch the liar. 

Point the same question twice differently noted his answer with reading body language

Being senior, one could easily trap him. The way he communicate and expressions are enough and at the end of the target is something gives you clear indication about his performance.

If there is any slight change when asked repeatedly with some technique in order that you are going to agree by him if understood clearly then we may have chance to know about the lie.

Dear Hanny,

thats foremost ,for one to keep a check on the sales staff and their body language,the words that they speak. Moreover being well informed about the market shall help us to an extent, alerting us by not being cheated .

More over marketing of a product ,mostly involves a lot of hype and push for the product, wether the product is good or bad.people can be really good at it and u cannot even make out if the person is lying or not! 

trust your instinct and knowledge.

At the time of sales process just follow your perception while after this process you can double-check in many different ways.

This Is Not Easy To Find Out . A Customer Who Has This Question Always In His Mind When He Is Going Shopping Will Not Have A Pleasant Shopping . Answer For This Is , If You Feel He Is Lying Then Talk To Him Directly And Openly About Your Doubt And Clear It Before Purchasing . 

Do Not Worry , Keep The Bill Safe And Inquire Him About The  Return And Exchange Policy . 

If Hes Complaining About His Company , Its Not His Company He Is Complaining About , Its His Bad Experience With Some Colleagues Or Some Personnel In His Company Or Management . This Is  Different a Subject .

First, through my experience and my vision emotions and logical extent of what to say when a sales person. Second is a practical follow-up to it, he disembark and question it when the client and clarify the extent of the sincerity of the lie.