Guy at work was being really nice to me a couple of weeks ago. Then, same situation as you, I find that he's talking to the other colleagues when he thinks I don't hear it, about how I'm a bit more feminine than the other guys at work. So now I just don't give him much mind, and avoid talking too much to him, simply because I think it was a rotten thing to do.
How can you know if someone is being honest? The truth is, you can never know. Never. That is the meaning of 'trust'. You believe that a person is trustworthy, but it could always happen, at any given moment in time, that the person will betray your trust. Maybe in weeks, months, even years after believing they can be trusted. That doesn't mean you should assume the possibility of betrayal in everyone you meet, but you must not let this turn your world upside down, and you must always be cautious when sharing confidences.
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clarity of thought before rashness of action