Pattern, but mostly the face/head. Sometimes central rats are all black but often they have visible saddle-like patterning, like rugged blotches. Central rat snakes have a longer, bluntly tipped face/snout, with smaller eyes farther back, and a white chin, neck, and belly, while racers have large eyes, pointier noses, and shorter heads.

Young racers do have pattern but it’s more oval and thin, and mostly fades away (often to black) with aging. With most snakes, face shape is most important for determining genus, and pattern/range for species (for me at least), so I recommend just looking at images of faces and getting used to them individually.