I'm sure everybody has their own way of doing this. Typically, checking your results after the first pass or when you are finished seem to make the most sense. For me, it's the mustache area. I found this out one day (with which razor I forget). I did a single pass (I usually do two), and realized I didn't need another. Everything was smooth, clean, and the second pass felt very unnecessary. When I feel this, I know I have an efficient or perhaps a VERY efficient razor. Also, the end result and how I got there in terms of passes/effort factor in as well.
What are your thoughts and how do you measure the level of efficiency you prefer in your shave?
integritas pietas fortitudinem