In our ongoing caffeine study, the dependent variable is test performance. For the dataset I made available here, the column labeled "score" is the dependent variable - score on the fictional test. The column labeled "group" contains the independent variable - whether that person was assigned to receive caffeinated (1) or decaffeinated (0) coffee.
One thing that is very important in establishing cause and effect is temporal order. Even if you can't manipulate the independent variable, you need to at least show the independent variable happened first and the dependent variable happened (sometime) after. While after doesn't necessary mean because of (a fallacy we call post hoc, ergo propter hoc, or "after this, therefore because of this"), if the independent variable didn't happen first, there's no way it could have caused the dependent variable. (So it's a necessary, but not sufficient, condition.)
So in conclusion: