Sometimes it's hard to follow your heart

Just how much credibility are there in someone's words?

How do you know whether he/she is trustworthy?

I'm not sure how trustworthy he is, or how much belief I can take his words, but am I wrong if I'm starting to doubt that whatever he promised me isn't out of sincerity in the beginning? 

Is it me who lacks of confidence, or is it me that don't believe someone like him?