How do you catch people at being insincere when they are talking to you?

People sometimes say nice things just to spare your feelings even though they mean the opposite. For example, they might say to you "you are my best friend" when what they really think "I don't really like you that much, but I feel bad for you so I'm gonna say I like you a lot." How do you catch people from being insincere? Do you have these doubts sometimes?