One of the many field marks that one can use to differentiate immature sharp-shinned and Cooper's hawks is the pattern of breast streaking. Cooper's hawks generally have crisp, teardrop-shaped streaks, with a clear differentiation between dark and light areas, so that the overall effect is of a light-colored underside. Sharp-shinned hawks tend to be more mottled with less distinct streaks, so that the overall effect is of brownish underside.
Of course, this characteristic should not be used in isolation since individuals vary a good deal. Still, it is a useful identification point in combination with tail shape, wing shape, flight pattern, head shape, or leg size.