Mechanisms for capture and digestion of prey vary greatly among the six (or more) lineages of flowering plants that have well-developed carnivory, and some are much more active than others.
Passive carnivory is common in some groups, and one, Roridula (Roridulaceae) from southern Africa, is so passively carnivorous that it requires the presence of an insect intermediate to derive any benefit from prey trapped on its leaves.
There is no clear way to discriminate between cases of passive and active carnivory and between non-carnivorous and carnivorous plants – all intermediates exist.
Ref.: Murderous plants: Victorian Gothic, Darwin and modern insights into vegetable carnivory. 2009. MARK W. CHASE, et al. Botanical Journal of The Linnean Society 161: 329 – 356.