There are no “proven” signs that a person is lying. In fact, the most effective liars of all are sociopaths (also called “psychopaths” or “anti-social personality”).
Sociopaths do not react to lying the way normal people do – in fact they have neurological abnormalities that cause their brains to react differently to the stress of lying. They also do not feel empathy with other people (although they can do a wonderful job of acting as if they do.)
Sociopaths are one reason that lie detector evidence is no longer accepted in court. Sociopaths lie like rugs and show no reaction on the machine, while a normally nervous person can show up as “guilty.”
Don’t even assume that you can tell if a person is lying. You might end up buying the Brooklyn Bridge or contributing to another Madoff. When in doubt – back out. Whether it’s a shidduch or a business deal, check it out carefully.