How do monarchs find milkweed?

Monarchs find milkweed using their sense of sight and smell (sensory receptors). They have sensory receptors in their antennae and front legs. Females will 'taste' milkweed with their feet prior to laying eggs on it. Reproductive female monarchs continuously move across the landscape in search of milkweed on which to lay their eggs. For a more detailed response, Journey North has an excellent article on the topic.