There are certain rules of thumb that you need to follow, the rest is experience which you can pick up quickly. I think it was already mentioned that a) you throw out the initial portion which is likely to contain methanol. b) you cut it off before you get the longer chained fractions in. c) you filter it and let it settle for a while. Some people never get it, but as a rule well done moonshine is miles better than the commercial stuff. Also it’s not hard to tell when it is.