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You don't'. Unfortunately, we can never know what's in someones mind or heart, we can only know what they tell us. The way you can "kind of" tell is by how he treats you and by waiting a good amount of time before committing to someone.
The only way you can tell if someone is honest is if you know them long enough to know that they can keep their word. Over time, you learn how someone treats you, and actions speak louder than words.
I welcome your thoughts, let me know if you want to talk more