What's the difference between "knowledge" and "full knowledge" when it comes to sin?

This is actually not a speculative question! :) An act committed with mere knowledge is not a mortal sin--it must be full knowledge. So, what is the difference between mere knowledge and the full knowledge required for a mortal sin? Where's the cutoff? I don't know if this needs to be a second question--what's the difference between mere consent, which does not result in a mortal sin, and complete consent, which does? What's the cutoff? The Catechism gives a partial answer: 1862 One commits venial sin when. . .he disobeys the moral law in a grave matter, but without full knowledge or without complete consent. But it doesn't give a cutoff point for knowledge or consent. What's the cutoff? When does knowledge become the "full knowledge" and consent "complete consent" required for mortal sin? And how could I be absolutely sure I crossed that line?