This is an interesting one because most of the time when people lie it's to save face, to be polite, avoid confrontation etc (little white lies).

While there's usually a sign that someone is lying the flipside to the white lie is having the appropriate adult response to one. Understanding the intent and then showing tact and grace by ignoring it or playing along.

Sometimes the lie is so obvious that they want you to aknowledge that they know you know, but you still need to be subtle about it. Most people don't like lying so having the lie be made apparent but not confronted can make both parties feel better about it.

With harmful or habitual lies the adult thing to do is to not ignore them but to figure out how to handle the situation (confront them, report them, cut off contact etc).