For some promotions (like spend y in x and get item discount) OOTB qualifier service doesn't mark any commerce item as qualifier. And evaluateQualifier() method returns boolean instead of List type.
So in these particular scenarios: how to find the commerce items which acted as qualifier?
Turn on the loggingDebug in PricingManager and PricingEngine. It will shed some light on how the pricing is working and what qualifiers are used.