How do you know he's bad?

You're sitting at home. Suddenly the door rang.You opened the door. An old man at the door.He said his car broke down.He wanted to help you.You should accept? Many bad people take advantage of the good faith of the people. But this "don't help" doesn't mean. Ask a few questions.If he's telling lie, he answers slowly. Or if he answers more quickly, that's mean he's telling lie. For example, you can ask he's car's problem. Or you can ask that when broke down. Accept help.If it shows too much response, it can be wrong something. Carefully look at his face.Is the edges of his eyes is wrinkled? Then no problem. But, is there a sly smile on his face? I mean, lips curled up from one side? That's can be a problem. Undo offered to help.And close the door. That's all.