14. If it looks like a duck, quacks like a duck, swims and flies like a duck, waddles like a duck, it is almost certainly a duck."
Which sounds similar to if someone tells you who they are believe them.
Taking that out of context, I don't agree. If someone tells you who they are, and they sound like a duck, quack like a duck, etc., then they are almost certainly a duck, despite what they told you. If they told you they are a duck, all fine and good. But if they said they were an eagle, you know....
Take the duck out. They told you they were there to help you, but took advantage of you or hurt you, then they were not what they told you.
The point is to use our power of observation but don't make too much out of or reject the obvious.