First, when a person shows you who they really are, believe them the first time. This is not to say that some people can't change, and that people won't make mistakes, but sometimes you can avoid a lot of trouble if you are able to cut people off in the very first moments you meet them, and notice their character probably wont help to influence your life in a positive manner. 

Second, we have to be willing to take a stand for ourselves. If a person continues to tell you one thing, and you notice that their actions aren't usually representative of their words, then you must take note of this, and use this as a warning sign that this person may not be who they really say that they are. 

Don't spend all of your time living confined, without making new friends, and taking new risks, but do be careful of whom you put your trust in, not everyone has your best interests at heart.