I argued that Epicurus sought to infer all truths about what cannot be perceived (what is "non-apparent", adêlon) by using what is perceived (the "phenomena") as "signs" of what is non-apparent. This is an empirical method, based on the claim that sensory perception shows what is true. In the case of his basic physical doctrines, Epicurus argued by a reductio ad absurdum that if the negation of the doctrine were true, there would be a conflict with what we perceive. This applies to Epicurus' first two doctrines, that "nothing come to be from non-being" and that "nothing is destroyed into nonbeing", as much as to succeeding doctrines. Contrary to the usual view (which goes back to Aristotle), Epicurus did not take over these claims from Parmenides. Instead, he proved them by opposing Parmenides' conceptualism with the empirical claim that the phenomena are true. I suggest that Epicurus derived this method from the early atomists.
Asmis, Elizabeth, "Epicurus' Scientific Method" (1976). The Society for Ancient Greek Philosophy Newsletter. 157.