What is an immediate red flag someone is not to be trusted?

What are some things a person can say about themselves that make you instantly assume them to be untrustworthy, shady people? We're all familiar with the paradox we live in, being social herd animals by nature, yet disliking the other members of our species. What little thing is an immediate red flag that you’re not going to like someone?
Some of us have friends. Others would like to have friends. For those who do have positive relations with other select people how do you determine their reliability and trustworthiness? How do you make sure they've deserved your friendship?