How do you know if someone is attracted towards you?

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How do you come to know it? What are the signs and hints that the opposite person conveys? Does the person show it also that he/she is interested in you? Some people are straight forward and directly come and tell you, but what about the people who are attracted towards you but don't tell you about it! They do express it as well, if not by words then by their body language, but how do you read their mind and their body language? I am really interested to know this!