Fruit flies one per fruit. In some Heliconus species female will check host for other eggs to make sure there aren't any. Insects attacking vertebrate hosts or predators and parasites have the same problems finding their food as herbivores. In general they use the same sorts of clues that plant feeding insects do but there are some differences, particularly for those species which feed on vertebrates. Nature of identifying factors somewhat different.
Hosts also have distinct odors but they also provide other clues such as heat.
In addition, the host is mobile while a plant has to sit still and take it. Sense direction of host and will pursue. The clues used are somewhat different from those used by a plant feeder. Again shape is important but color is not. Movement is an important cue as movement readily distinguishes an animal from its background.