Full Idea
We can unambiguously differentiate the cause from the effect in that whatever stimulates or releases the action of the powerful particular involved in the causal production is the cause or part of the cause of that effect.
Gist of Idea
The cause (or part of it) is what stimulates or releases the powerful particular thing involved
Source
Harré,R./Madden,E.H. (Causal Powers [1975], 6.IV)
Book Reference
Harré,R/Madden,E.H.: 'Causal Powers: A Theory of Natural Necessity' [Blackwell 1975], p.114
A Reaction
I have doubts about distinguishing stimulus from release, and they sensibly don't say they have a test for 'the' cause, but I roughly agree with this idea. I take 'the' cause to also be tied in with explanation.