Leaf: Simple, alternate on stem. Black willows have very narrow leaves with finely toothed margins and are shiny green on both sides. Peachleaf willows have long, pointed, lance-shaped leaves with finely toothed margins that are whitish underneath and borne on long, slender, somewhat twisted stems.
For more information see
http://www.dnr.state.mn.us/trees_shrubs/deciduous/willow.html.