Consider the source. Determine where your resources come from and whether you're taking more than can be replaced, either through natural processes or your own practices.
Where are your resources and inputs coming from? Think specifically about water, energy, soil amendments, and feed (if you have livestock). Also think about long-term, capital investments, such as structural building materials, tools, etc.
Keep in mind that no farm is an island: complete self-sufficiency is not a requirement of sustainable agriculture. Long-term stability and productivity is. The more renewable and varied your resources are, the longer your farm will last.