ConstructionDecisive feature
MetaphorNonliteral use through resemblance; a blocking clue
MetonymyNonliteral use through another association
AllusionImplied concomitant; literal sense remains possible
“Asad delivering a sermon,” when it means a brave man, is a metaphor. “Much ash,” when used for generosity, is allusion. An unusual-looking expression is not enough: identify the association and interpretive clue.