Sometimes when my conscience is pricked that a particular action might be a sin, yet I am determined to brush over it and convince myself it’s “not that bad”. I ask myself, “Am I in anyway trying to hide this particular action from someone else?” (i.e., I turn off the computer when a family member comes in the door so they don’t know how long I've been sitting there; I cover up the contents of my garbage can at work so no one knows how many candy bars I've eaten that day, etc.). I have found, in my life, when the answer to that question is “yes”, then I know it’s sin, thus my desire to hide it.
It is a practice going back to the first sin in the Garden of Eden.
And they heard the sound of the LORD God walking in the garden in the cool of the day, and the man and his wife hid themselves from the presence of the LORD God among the trees of the garden. But the LORD God called to the man and said to him, "Where are you?" And he said, "I heard the sound of you in the garden, and I was afraid, because I was naked, and I hid myself." He said, "Who told you that you were naked? Have you eaten of the tree of which I commanded you not to eat?" Genesis 3:8-11 (emphasis added)