Knowing the difference between need vs. greed is a matter of conscious discrimination. We all have needs that must be met for our physical, emotional, psychological, spiritual and economic well being. Our needs are what they are: they must be fulfilled, or we simply cannot get succeed and get by in life. However, we say that desire is excess: it always wants more than is necessary. It always seeks to go beyond, to the exorbitant, to the exaggerated. Where needs end, covetousness begins, which seeks to accumulate for the sake of accumulation, not because it fulfills a necessity so that our life can function.
It is true we need money, a home and place to sleep, water and food. But when that becomes excessive, exaggerated, more than what you utilize on a daily basis and more than you will ever need, we can say that the secret triggers of such behaviors in accumulation and hoarding are rooted in desire. For example, you need a car to get to work. But ten cars? You need a house or apartment in which to live and sustain yourself. But five? You need money to buy your necessities, but millions of dollars that are saved while people around you starve?
Eating organic is not necessarily driven by desire, but it can be. It is necessary for us to eat healthy as much as possible. In fact, we recommend that you do. But it is important not to make diet one's religion. It is necessary that our diet reflects and assists our religion, and not that our spiritual discipline exclusively consists of regulating what we physically eat.
You will come to know what is need and what is greed through meditation. You have to analyze, with a concentrated mind and heart, all of your necessities and all the extemporaneous things in your life. This analysis depends upon the scrutiny and discriminative understanding of the
Consciousness, directed with one-pointed concentration. Lesson Five from
Introduction to Gnosis by Samael Aun Weor might assist you in comprehending this subject. It provides a meditation you can work with in order to learn how to discriminate between need vs. desire.