How do I know if I committed the sin of lust?
Sin is in the will. Feelings and thoughts that pop into our minds aren’t sinful in themselves. It’s when we entertain, consent to, or nurture inappropriate thoughts, feelings, or passions, that we sin.
How we know if they are “inappropriate”? Are you having sexual thoughts about someone you’re not married to? Are you thinking of someone (married to you or not) as a piece of meat, or otherwise an object for your personal enjoyment?
How do we know if we’ve “entertained” them? Once you became aware you were having inappropriate thoughts, did you give in, or decide to keep thinking about them?
How do we stop entertaining them? Calmly turn your mind to something else. Don’t stress out, don’t try to fight it, just distract yourself with something else which is good or harmless and absorbing.
I am a married man and so for me it is easy. I find a strange woman in a place where only my wife belongs.
My experience in the confessional has brought me to the place in my life, and it took a long time, to realize that these types of questions generally are best discussed with a priest. Your true intent, your free will choices need refining and by that I mean that we all need to ask for guidance when we consider our potential to sin. That is best done in a confessional with a priest.
Choosing to take some kind of sexual pleasure in a woman who is not one’s wife. It could be checking out a woman walking down the street, or a woman in a magazine or on the internet. Undressing with the eyes. Certainly enjoying porn is always lust. It could be imagining having sexual relations with a woman whether it’s one you know of, or an imaginary woman.
But remember it has to be a choice. If a thought occurs to you and you realize you have a choice to make and you go ahead and do it anyway-- that is the sin. If a thought occurs to you and you reject it, there is no sin. It’s only a temptation. Pray when temptations come, and go to confession if you have trouble with giving into such thoughts or looks.
Taken from a very reputable book from the 19th century called The Way of Interior Peace by Fr. Lehen, pg. 99:
“To a mortal sin belong three points: 1st. Weighty matter; – a jesting lie, for example or a vain, self-conceited thought, is not sufficient for a mortal sin. 2d. Full knowledge of the evil; that is, the deliberate consciousness that what one does is a mortal sin. This excludes all cases in which the sin proceeds from surprise, or in which the soul is not full master of her powers; as, for example, in half-slumber, and the like. 3d. Full consent of the will to that which the understanding knows to be mortal sin. As long as the consent remains imperfect, or we are conscious of a certain hesitancy, a deferring, or a reproach of conscience in consequence of our neglect in combating the temptation, the sin is only venial.”
I hope this helps.