EWTN Catholic Q&A
Question from Jason on 11-06-2012:

When you have committed mortal sin do you know that you have sinned? I am not sure if I have committed mortal sin from somethings that I have done. I

Answer by Fr. John Echert on 11-06-2012:

If the sin is of a very serious nature, was done with deliberation and with knowledge that it was serious, then it probably was a mortal sin. There are some people, however, who are scrupulous, meaning that they exaggerate the seriousness of some sins, but that is another matter. If in doubt, make a good confession the next chance you get and the priest will sort it out for you.

Thanks, Jason

Father Echert