Look at the front of the skull, the rostrum region, where one of the major (though subtle) distinctions lies. The nasal bone is the uppermost surface; below, there are the premaxilla and maxilla, the lateral and ventral bones of the rostrum, respectively. These are the frontal, jagged wedges of bone above the jaws. In bison, the nasal and premaxilla are well separated by the maxilla and space; in cattle, the two bone segments often touch, or nearly do. Whether premaxillary touches nasal bone is an important clue when identifying the skulls of ungulates, many of which, superficially, closely resemble one another.
Consider the skull's size. This isn't a foolproof rule, especially given the variety in cattle sizes, but bison are typically more massive than their domestic brethren, especially U.S. cattle breeds. A big bull bison might weigh more than a ton, and is characterized by an enormous head and huge, burly shoulders. The formidable anterior (cows are also more heavily constructed to the front, though less dramatically so) is an adaptation for sloughing snow off in winter browse and--among the males--for head-butting battles.
Look at the horns. Those of bison are always thick and sharply curved upward. Steer horns exhibit more variation. Consider the great, elongate, eponymous headgear of the Texas longhorn, for example, which might reach eight feet tip to tip. Then there are horn-less species of cattle, such as the Angus. Both male and female bison always exhibit horns. Among the bovids that possess them, horns grow throughout the animal's lifespan and consist of a bony core and an overlaying sheath of keratin. In the oldest bison, the tips might break through the sheath so that the ends of the horns look somewhat blistered.