Shatter
That is s great question. I have been taking samples of the drips of butane during the process and letting them dry. It takes a few seconds for a drip to dry. If it leaves a residue, then it is not ready.
It is a qualitative measurement and is not a good way to measure, however, I did a sample test to quantify if my method was leaving behind material. I ran a sample batch using 6 cans (after one of the runs). I collected the material and boiled off the butane. The extract was 3 grams. Considering the cost of butane and my time, it was not cost effective use the extra butane to extract beyond my verification method.
RE: Crude Method for Cannabis Extraction