Tune Into your feelings - when you feel discomfort or resentment during an interaction or when you're in a situation, ask yourself what is causing those feelings to come up.  Resentment usually comes from feeling taken advantage of or not feeling appreciated.  This can be a sign that you are the one pushing yourself beyond your own limits because you feel guilty or someone else is imposing their expectations, views, and values on you.  When you feel uncomfortable, its a cue that someone is crossing your boundaries.  

Practice self-awareness: If you feel resentment or stress, its your first cue that you have been pushed beyond your limits or your own boundaries have been crossed.

Consider your past and your present: How were you raised as a child?  What role you played as a child will greatly influence your ability in setting and sustaining boundaries.  Take notice of the people you surround yourself with.  Do they take more than they give?  Are your relationships reciprocal?