How to know when a sin is mortal sin

Question from RAE on 9/27/2007: The catechism seems to leave the definition of "mortal sin" rather open. I understand the 3 conditions of mortal sin (grave matter, full knowledge, and deliberate consent) but how to know the degree of gravity is left rather undefined. We are told though that if we have committed mortal sin not to take communion and go to confession but how can we know if we are in state of mortal sin? I am sure it cannot be left only to our conscience to discern... Answer by Fr. Robert J. Levis on 9/28/2007: Rae, The Church for centuries has taught the grievousness of some sins, their objective morality, in her catechisms, papal lectures, books, etc. The individual Catholic must take it as his personal responsibility to know these things, and to use prudence in assessing the three standard requirements for mortal sin: serious matter, sufficient reflexion, and full consent of the will. When in doubt, he can always inquire of his priest. Fr. Bob Levis