Scab is identified by the appearance of premature white heads.
Loose smut causes the head to turn black and appear to have been burned.
Sca b can attack a single spikelet, a portion of the head, or the entire head. Smut typically causes the head to turn black. Also, with scab, you can probably detect the orange spore masses now developing at the base of the glumes. Champaign has scab and is showing the spore masses. You can see these easily without a hand lens.