Caterpillars hatch out of eggs which are laid on the leaves by butterflies. So examine the leaves for eggs (groups of small white or yellowish spheres). To keep butterflies off low growing plants you can cover the plants with mesh.Don't know if this would be practical with your beans though. Otherwise just be vigilant and destroy any you see. A few holes in leaves will not affect the crop, but you need to avoid an invasion! I've never seen caterpillars on beans.