Look for their food source!
Adult monarchs need all kinds of flowers to feed on. That means that you can spot them anywhere that nectar-bearing flowers are growing: in meadows, along roadsides, in gardens, etc.
During their fall migration, monarchs are often seen on goldenrod (Solidago spp.), asters (Aster spp.) and liatris (Liatris spp.) and in fields with clover, sunflowers and alfalfa in bloom.
An adult butterfly’s main goal in life is to reproduce. Females have to quickly locate their host plant, milkweed, in order to lay their eggs. So, during summer, monarchs can be found wherever there’s milkweed growing!
The northern limit of milkweed’s geographic distribution is below the 50th parallel. Very sunny, disturbed sites are well suited to milkweed (ditches, fencerows, abandoned fields, open areas, shorelines, etc.) and are thus a good place to look for monarchs.