Although Poulantzas does not give a direct answer to the question 'How are we to know a pertinent effect when we encounter one?', he does discuss an historical example which may throw light on the matter.

Suggested, for I admit that our present position is completely agnostic in respect to it, since we have no evidence which throws light on the matter; and study of the subject would appear to be mere waste of time.

Anyhow, as he was away this weekend, I decided, on the off-chance that he is in communication with some no-goods and that I might find something that would throw light on the matter, to search his quarters.'